lack of power

I have a 1988 Mazda 626 LX with a 2.0 Liter V4. Every once and a while when I press on the accelerator, the car losses power and eventually comes to a stop and shuts off. Also while the car is losing power I hear a hissing sound coming from the right rear side of the car. When I turn the car back on it runs fine for awhile (a couple of days) and then the same problem comes back.

Does anyone have any suggestions?

I found the answer. The culprit was the gas filter. I almost was abused by a mechanic who is part of one of those big automotive stores. He claimed that the head gasket had to be replaced and the valves needed to be worked on. Luckily a friend of mine sent me to his mechanic who found the right answer.