Ford escort zx2 2002 coupe not fully accelerating, hesitates and then clicks and goes?

My ford escort zx2 2002 coupe has been driving fine for a while, only 55k miles on it and recently got some pipes and a vacuum tube fixed. But recently I've been driving it and sometimes on take off my cars rpm will go up a bit and it feels like it is hesitating to go, and it stays like that for 1-2 seconds and then clicks! It clicks with the rpm and drives smoothly. It's happened before and my dad said it was from a dirty spark plug or filter, but that was 2 months ago. What would you think the problem is?

And the engine light is not on!

It is something in the transmission. It slips causing the engine to rev before engaging. I can't be 100% sure, but that's what it sounds like.