My girlfriend's car - 4cyl auto - had a leak in a head gasket (oil was flooding one of the spark plugs and car was running on 3 cylinders). The car wouldn't pull at all until it accelerated to 30+ mph. I drove it to mechanic - he replaced head gasket, put 4 new spark plugs and new spark plug wires, as well as new fuel filter as maintenance. He also reset the computer.
I picked up the car - it was driving fine at first - then started rumbling and hesitating when gas pedal is pushed. Now you have to really apply gas slowly or the car shakes and almost stalls (no power). What the heck can it be???
I think maybe the fuel pump? I took the car back and mechanic has put in a different set of sparkplugs, new sparkplug wires (again), and another fuel filter. Gasket doesn't leak oil anymore. But this sh*t seems to go on constantly and spoils the driving a lot.