Running problem with 89 GLI

My 89 Jetta GLI started acting very strange at stops recently. After I drive it about a mile, when I come to stop and let it idle, it will drop down to 400 RPMs and then rev itself back up to 1200 or so and continue to do this a few times. Once it gets up to temp then it stops doing that. I have also noticed that when I turn on the a/c it will stop it as well.

Also another problem, occasionally it will run like it has no power. I ride dirt bikes and it reminds me of a dirt bike being on choke. Maybe the oxygen sensor?

Any help is appreciated, thanks

ck exhaust, You may be having back pressure problems. Mine does this at times.

what do u mean by back pressure problems

Clean your idle stabilizer valve with carb cleaner. It's a very common problem.

Clean your idle stabilizer valve with carb cleaner. It's a very common problem. As far as the loss of power is concerned, I'd check your fuel pressure.