Engine Performance problem
1999 GMC Sierra V8 Four Wheel Drive Automatic 273000 miles

Sorry guys-you have the wife of a very frustrated guy here. I have to ask, as he's at his wit's end, so please bear with me...
Bought the truck 2 weeks ago-(5.9L Sierra 2500), and on the way home up a hill in overdrive it revved out when his foot was on the pedal, but no acceleration, almost like it's in neutral.
When started, it idles low (500rpm) then when put in gear it wants to stall. It accelerates OK otherwise, but is running rich.
Code reader said P0122, so he changed the TPS, & a blown 40amp fuse, cleaned battery terminals, changed plugs and wires, fuel filter, IAC, cleaned the throttle body, MAF, and EGR. He's checked for a vacuum leak, and voltage to TPS.
When he disconnects the TPS and IAC and starts it, it idles just fine for a few minutes, then idles low again.
Thanks,
Coley

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Service Information
1999 GMC Truck GMC K Sierra - 4WD | Sierra, Silverado (VIN C/K) Service Manual | Engine | Engine Controls - 4.8L, 5.3L, and 6.0L | Repair Instructions | Document ID: 392494
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Idle Learn
Idle Learn Procedure
Anytime the powertrain control module (PCM) or the battery is disconnected, the PCM loses power, or the PCM is reprogrammed, the PCMs learned idle position is lost. The engine idle is unstable when the learned idle position is lost.

Perform the following procedure in order to return the learned idle to the correct position:

Automatic Transmission
Turn OFF the ignition.
Restore the PCM battery feed.
Turn OFF the A/C controls.
Set the parking brake and block the drive wheels.
Start the engine.
Ensure that the engine coolant temperature is more than 80°C (176°F).
Shift the transmission into Drive.
Allow the engine to idle for 5 minutes.
Shift the transmission into Park.
Allow the engine to idle for 5 minutes.
Turn OFF the engine for 30 seconds.
Manual Transmission
Turn OFF the ignition.
Restore the PCM battery feed.
Turn OFF the A/C controls.
Set the parking brake and block the drive wheels.
Place the transmission in Neutral.
Start the engine.
Ensure that the engine coolant temperature is more than 80°C (176°F).
Allow the engine to idle for 5 minutes.
Turn OFF the engine for 30 seconds.
© 2008 General Motors Corporation. All rights reserved.

He tried it, still the same. (BTW, the truck is a 2500 SLT, not a Sierra, if it makes a difference.)
Any other ideas?

What engine does it have?

6L

What is the TPS voltage with the throttle closed?

Sorry, had been reading way too many posts when I first asked the question-you do have the right info, promise! He's doing a compression test today. He's in the industry, so he's, talked to a few people, but this one is really stumping him. He's tried all I've suggested on this site, (after reading till my eyes are crossed) and now the only other thing I could think of was maybe the 02 sensor? I wonder if he has 2 separate problems, the low idle (no black smoke, just smells rich) and the overdrive revving out like that. I had a similar problem with OD in my truck, but it was the catalytic converter, no idle problems.
I was thinking fuel pump or pressure, but he said they had it on the big scanner at work (forget the exact name) and it would have checked the fuel pressure, and that no codes came up after changing the TPS. And still that low idle, wants to stall in gear, and we are still confused.
He called the guy he bought it from, and he said he never had a problem (of couurse) except for one time when it was really hot out, it was running rough.
Only other thing I can think to tell you is that is has that engine tick, which I've read is common.
Any ideas? (His are to burn the ***thing)

I believe it was 5 volts.

Check for a leaking fuel pressure regulator. Remove the vacuum hose and see if it has fuel in it.

Fuel pressure reg. is OK, and compression test was OK.