I have a 1990 subaru legacy ls with the 2.2 and 4eat tranny with power/manual modes. Lately when i have been driving the car i have noticed a lag all of a sudden at highway speeds. i will watch the rpms and i will notice the engine will drop about 100 to 200 rpms quickly... kindof like someone pushed in the clutch on a manual tranny Today I checked the wires, coils and such, replaced the fuel and air filter, pvc valve, and also gave it an oil change. Today it was doing it like 3 or 4 pulsating times. I can always regain the rpms but its kindof a pain out here with some mountain driving or denver driving. I also notice it on normal driving, anytime the car is at 3000 to 4500 rpms The car is not throwing any codes and will do the same thing in manual mode, doesnt do it in power mode, but thats hard to get the car into that... I cannot drop the rpms by verying the gas that fast... I have not changed the timing belt since i have owned the car, bought it last year at 159,000 now at 180,000. any ideas?

i drive car for work today... and it never did it... did it last night when i shut the car off... now, will it do it again tomorrow

it seems to do it more often after filling up with gas, its now about 50 rpms...

I also notice it on normal driving, anytime the car is at 3000 to 4500 rpms The car is not throwing any codes and will do the same thing in manual mode, doesnt do it in power mode, but thats hard to get the car into that... I cannot drop the rpms by verying the gas that fast... I have not changed the timing belt since i have owned the car, bought it last year at 159,000 now at 180,000. any ideas?

That sounds pretty much like a knock sensor issue. Do a search for that for more info. . .