essial

This one's a little strange, I just need someone to point me in the right direction. Sometimes (usually when i'm sitting at a redlight) my car will start idling funny. It will dip below 900rpm (which is where it normally idles aprox.), then start going up and down up and down in rpms. When it does this, pressing the gas pedal has almost no effect on the rpm. This even happens when the rpm is above 1000.. it still will not accelerate quickly, it will slowly speed up, and start shaking worse and worse. Sometimes it does this while moving also.

I have replaced:
* Spark Plugs and spark plug wires (car has no distributor cap)
* Air Filter
* Fuel Filter

I tested the mass airflow sensor by unplugging it, which I read should cause the vehicle to entire a "safe" mode, and should still run. When I unplugged it and crancked it up, it'd start, then slow down and shut of (just one smooth rev, then rev back down to 0). I don't know if that helps any.

When the above thing happens, theres no smoke or weird smells (no fuel smell or anything). Also, no check engine light comes on.

Any ideas?

The car is a 1993 Ford Escort 4cyl 1.9L manual transaxle.