Why does my car not accelerate properly?

I have a '99 prelude, and for the past few months it hasn't been accelerating properly.
When I start my car it drives normal for a bit then the RPM drops.And I have to keep
revving for it drive forward.It goes up on the RPM then drops suddenly to 1 and then i
have to push hard on the accelerator for it to jerk forward  After driving for awhile (15
minutes or so) It finally starts driving normal. I always have at least half a tank of  gas
and it still does this. I've taken it to an auto place two times both times they've said the
"fixed the problem" but a day or two later it starts happening again. I can't afford to buy
a new car right now so I hope the problem can be fixed. I love my prelude :(