RPM drops
 

my car is 93 lancer glxi. a month ago my car got overhauled bcoz of some sounds in the engine. recently, i'm experiencing some problem with the rpm which will suddenly drops, it happened twice when i switched on the ac but i also experienced the flactuation even without the aircon turned on. And when you step on the gas pedal, the car wont move forward and the rpm wont increase as if no gas was being supplied. It's kind of the engine will turn off, but it's not, its just that the rpm is too low, perharps around 300-400 rpm. The normal rpm of my car when idle is 1000 but when this problem occurs, it will go down to 800 and even 400. I went to my mechanic and he said that it could be the fuel pump. But he adjusted first the "servo" (?) and said to observe my car. what could be causing this problem. can someone give an advise. thanks.