99 Pontiac GTP cutting out and won't rev

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Hi all,
Hoping to get some advice from people that know these cars.
After reading a lot of the posts here, I can tell there are a lot of knowlegeable people involved with their cars and turning wrenches.

Anyway the car's a '99 Pontic Grand Prix GTP with 72,000 miles and not a lick of trouble since new.
What's its doing now is, once it warms up, the idle will suddenly drop a couple hundred rpm and sometimes it just dies. Mostly it will cutout briefly and recovers. This will happen every 30 seconds or so. When its running like that it will not rev over 3500 rpm without MASSIVE cutting out/missing. When it not in this "malfunction mode" it will run just fine and rev to the red with no loss of power or smoothness.
So anyway, here's where I am:
Mostly runs good until warmed up, not always though.
No trouble light on dash.
No trouble codes retreived (with borrowed OBDII tool).
Checked ECT, ohms O.K.
Checked IAT, ohms O.K.
Cleaned IAC
Cleaned throttle body
Cleaned MAF (with electonic contact cleaner only)
Replaced fuel filter
Replaced air filter
New plugs
New PCV
Replaced front O2 sensor
Tried a couple of fuel treatments to remove possible water, none found.
Checked for vacuum leaks .

Didn't know how to check MAP sensor, throttle position sensor or crank position sensor(s) or even the PCM.

Also one time during one of these episodes, the traction control light just came on,
for no apparent reason and I couldn't turn it off using the off/on button. After shutting the engine of and restarting, that symptom went away and hasn't repeated.

Thanks for any help.
John

posted by  tjschul

I would replace the MAP sensor due to the amount of trouble that GM have with them.the fluctuating idle seems to be a common thing.and the map sensor is not a ton of money.if you leave it too long,with a bad mixture that is too rich ,you will ruin the catalytic converter.and thats expensive.take the car and have a scan done and see if the map code comes up. OR TRY THIS.my favorite stuff is freeze spray.it is available in electronics suppiers.spray the map sensor ,once the car heats up and the problem shows.give a 5 second shot of freeze spray until frost appears on the sensor,if the problem goes,you found it.heat it with a blow drier to heat it up.and when the problem shows,freeze again,to confirm.....

posted by  the lobster

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