Problem, engine RPM "hops"

[Stallion]

This just started happening today. I first thought that it was a problem with the transmission slipping, because it felt like it would slip then regain and keep doing that. But then I put the car in neutral and kept the engine at about 2500 RPM. Sure enough, after a few seconds it would hop between 2000 to 3000 RPM for about 3 - 5 seconds and then keep the desired 2500 RPM again. This happens when driving, but it doesn't seem like there is a certain pattern to it. Just happens every once in a while. Also, the "check engine" light turns on when it happens every once in a while, but then after the 3 - 5 seconds of engine RPM hopping, the "check engine" light turns off.

What do you think this is? It's so inconsistent, doesn't always happen under certain driving conditions.

 

[Stallion]

I think I put this in the wrong part. I'm sorry. This isn't my '96 Corvette. This is my '94 Mercury Villager (automatic transmission). But please, any help would be much appreciated.

 

[wishuwerehere82]

OK I'm going to cut and paste some OBD1 links to help you read your codes, so hopefully you will be able to get the computer to help you find the problem.
http://wiki.answers.com/Q/Where_is_the_OBD_diagnostic_plug_located_on_a_1994_Mercury_Villager
http://www.articlealley.com/article_48352_31.html
http://www.auto-diagnostic-codes.com/plugins/Ford-US-EFI/FORD-US-EFI-67-Mercury.html

And finally, the codes themselves.
http://forums.probetalk.com/archive/index.php/t-1701015446.html

Although it's not a Vette, the computers work the same.
It could be O2 sensor, Mass airflow sensor, Throttle position sensor, or what Ford calls an Atmospheric pressure sensor. Or possibly the fuel pump.