2000 Chevrolet Astro

Engine problem
2000 Chevy Astro 6 cyl All Wheel Drive Automatic 106000 miles

I have had this P0300 random misfire code turn on and off for the last 2 months now. It came on in July 2007 and I changed the plugs, wires, coil, cap, rotor, fuel filter, EGR valve, IAT & ECT sensors. It went away for a few months, but now its back. Again, one day it's on (hesitating) the next day off and runs great.
P0300 comes on when the van hesitates. I also notice AVG fuel econ drop to ~12MPG when running rough, otherwise ~18 MPG when running fine.
When P0300/hesitation is acting up, it starts up quickly and idles smoothly. But at a constant 3000 or 4000 RPM, you feel the " hesitation/misfire". If you accelerate, you have power. Other details, humid days seem to trigger the P0300 more than dry days.
When hesitation exists, I checked for a vacuum leak by spraying carb cleaner near hoses and the manifold. No change in engine RPM.
When hesitation exists, I’ve tapped the MAF sensor. No change in engine RPM.
When hesitation exists, I tried disconnecting the MAF sensor. The engine RPM dropped, and then went up to a slightly higher RPM than normal. Plugged the sensor back in and the idle seemed smooth again.
So now I’m at a point where we start looking at replacing the fuel injection system. Quite expensive and may not be the problem. If the fuel injection system is the problem, I don’t see why the van runs so good some days?
Would it be possible that the EVAP, TPS, IAC, or MAP sensors could cause this P0300/hesitation problem like I described?
Last, other work done to the van; fuel pump at 50,000mi and the transmission was rebuilt July 2007. The AWD light came on before transmission was fixed. They changed the transfer case fluid and the AWD was working until the last 2 months. Anyway, I’ll save that for problem for another time, but thought I’d mention it in case there may be some type of sensor correlation.
Any suggestions to the next step would be appreciated, please. I want to keep the van for a few more years.

J. Zaremba
December 21, 2007.

Spray a saltwater solution on your plug wires around the plugs and distributor cap with the engine running. Look for any arcing from the wires. You might also want to remove the distributor and check the gear.I have seen many wear out and cause problems.

Dec 21, 2007.
Thanks' for the tip.
I did find arcing, but under the cap. It could be seen under low light conditions. Replaced the cap, rotor, and plugs again! This time with Napa brands. So far no problems and no service engine soon light.
Thanks' again for the tip.

Any suggestions on the service AWD light. Last time I checked the code, it was something about an encoder problem. The company who serviced the transmission also replaced the transfer case fluid. He used the special GM fluid for the transfer case. Mechanically, everything looked fine, and the service AWD light stayed off for a few months.
Well, the service AWD light is back on again. Now that we have some snow, I can tell only the rear axle is moving the van. Here's the thing. If you lift the van, and the tires are free to rotate, the AWD is working.
Is there some sensors I can check? If I have to replace these sensors, how involved is it? I have a Haynes manual for the Astro van. It does not cover this subject. Any suggestions on a manual that would cover this subject?
Any suggestions would be appreciated, please.

J. Zaremba

Jan 2, 2008.