Why does my truck idle too fast, then to slow?

Check ignition timing and compression. Check fuel pressure. Have it scanned for soft codes and live data. Soft codes won't set a CEL, so some shops won't try anything if they don't see it, so you need to tell them soft codes.

Smoking the motor checks for vacuum leaks. You can come close to the same results with a can of carb cleaner, usually. When the leak is found, the idle will smooth out. It may not be an intake or head gasket. It could be a hose, plastic line, or fitting. How well does the throttle shaft fit in the throttle body? Is there any play in it?

Did the shop just check for hard codes (setting the CEL) or did they check for soft codes, as well? What about polling live data?

Your statement about nothing happening when you unplugged the IAC made me sit up. I'm going to ask one of my colleagues to peek in here and give his opinion, but I believe that maybe the IAC is bad. Please have patience, as he may take an hour or two to receive the message and have the time to check in.

My opinion is a manifold gasket leak or fualty injector. But this is the way to set the IAC up correctly.
THROTTLE BODY INJECTION
1. Remove air cleaner and gasket. Plug TBI unit vacuum port for THERMAC (Thermostatic Air
Cleaner). With Idle Air Control (IAC) valve connected, ground terminal "A" to "B" at ALDL
connector.
2. Turn ignition switch to "ON" position, DO NOT start engine. Wait approximately 30 seconds for
IAC valve pintle to extend and seat in throttle body.
3. With ignition on, disconnect IAC valve electrical connector. Remove ground at ALDL connector.
Using an awl, remove plug for minimum idle speed adjustment by piercing, then applying
leverage.
4. Start engine. Adjust idle screw to specification. Turn ignition switch "OFF". Reconnect IAC valve
electrical connector. Install air cleaner and gasket.
SLOW IDLE SPEED (RPM)
NOTE: Adjustment is necessary only when throttle body assembly has been
replaced.

Thanks Merlin.

Happy belated Merry Christmas and a Happy New Year 2012-Boston Mike and nice to see you again old timer-

My opinion sounds more like the IACV is defective or its wiring back to the computer-who knows could very well be the computer-also might be stuck partly open or operating on its default values

Unplugging the IACV should caused the idle speed to react its purpose is to target the idle speed at all times at curb idle speed or you can adjust for minimum air as suggested by merlin

If its not set up right it may not work. That's why I included the instructions too set it up!

Thanks guys.

Get the smoke test. I still think injection or intake gasket. Injector should have fine spray, no drops of gas.

Has the TPS closed and wide open throttle voltages been checked/tested if not check it

Jacob, look for a small screw on the throttle body that stops the throttle from closing. That screw is the idle speed screw, or idle air adjustment. It has a plug over it, covering the screw head. It works in the same way that the idle speed screw does for carbureted engines. I can't find any images of the throttle body to help.

Here's the complete list of causes for poor idle.
ROUGH, UNSTABLE, INCORRECT IDLE OR STALLING
1. Check ignition timing. Check Park/Neutral switch circuit. See PARK/NEUTRAL SWITCH
DIAGNOSIS chart in GENERAL MOTORS COMPUTER COMMAND CONTROL article in
COMPUTERIZED ENGINE CONTROLS section.
2. Check injector for leaks. See DIESELING, RUN-ON. Check fuel pressure. See FUEL SYSTEM
PRESSURE TEST. If idle is rough only when engine is hot, perform the following checks:
Check for vacuum leaks by installing Plug (J-330247) in idle air passage. If closed throttle
engine speed is above 650 RPM, locate and correct vacuum leak.
Check Park/Neutral switch circuit. See PARK/NEUTRAL SWITCH DIAGNOSIS chart in
GENERAL MOTORS COMPUTER COMMAND CONTROL article in
COMPUTERIZED ENGINE CONTROLS section.
Check for sticking throttle shaft or binding linkage that may cause a high TPS voltage (open
throttle indication). If binding occurs, the ECM will not control idle. Check TPS voltage.
Reading should be less than 1.2 volts with throttle closed.
Check EGR valve operation. See appropriate EGR SYSTEM CHECK chart in GENERAL
MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE
CONTROLS section.
Check battery terminals, cables, and grounds straps for loose or corroded connections.
Erratic voltage will cause IAC motor to change its position, resulting in poor idle quality.
IAC valve will not move if voltage is below 9 or greater than 17.8 volts.
On 2.5L engine, check power steering pressure switch. See POWER STEERING
PRESSURE SWITCH chart in GENERAL MOTORS COMPUTER COMMAND
CONTROL article in COMPUTERIZED ENGINE CONTROLS section. ECM should
compensate for power steering loads.
Check MAP sensor operation. Start and allow engine to idle. Disconnect MAP sensor
electrical connector. If idle improves, substitute a known good sensor and retest.
Check A/C for high refrigerant pressure. Check A/C compressor or relay. If inoperative, see
appropriate A/C CLUTCH CONTROL DIAGNOSIS chart in GENERAL MOTORS
COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE
CONTROLS section.
Inspect oxygen sensor for silicon contamination from fuel or improper use of RTV sealant.
Sensor may have a White, powdery coating, and will result in a high but false signal voltage
(rich exhaust indication).
Check PCV valve for proper operation by placing finger over inlet hole in valve end several
times. Valve should snap back. If not, replace PCV valve. Perform compression test.
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Tech1 Question: hard starting hot or cold check engine light stays on intermitant

Fuel gauge fluctuation with fuel sloshing is normal. It means the sending units are reacting properly and immediately.

"Will start, then stall until fuel is applied."
If the engine doesn't want to stay running at idle, and everything else is right, this is usually an idle speed setting. Perform that idle speed adjustment mentioned above and it should fix the problem.