intermitent emission light

I have a 94 jetta, 2.0. auto.
The emission light comes on intermitently, will last for 2 to 5 minutes,there is a loss of power when light comes on, then car runs normal after light goes out. Replaced plugs and air filter.changed leaking valve cover gasket and changed oil and filter. Any help appreciated.

I'd look for a vacuum leak first. Also try cleaning your mass air flow sensors. The emission light is probably due to a too lean or too rich condition which is why you loose power. Sometimes the error is stored even if the light goes out. Take it to autozone and have them read your OBD for you. It could also be a fuel filter or pump. Once you know the error you should have a clue where to continue.

thanks doug
this is my sons car and just got some more info. He says the rpms go way up when the light goes on. Also does evey auto zone read the obd for you?

I think that the 94 is OBDI so the OBDII readers that are common won't work. The Advance Discount autoparts near to where I work has the appropriate adapters to connect up, but I wouldn't count on just any store having one. It's hard to say what's wrong based on the symptoms, but it doesn't sound like fuel pump or fuel supply problems. I'd look for vacuum leaks and loose sensor wires. The ECU may be getting bad data from one of the sensors. At this point, it's not much more than a guess.