Title explains it all. At a stop sign, on occasion when I release the break and press the gas, the engine will rev a little bit and go nowhere then jump forward. This problem is really bugging me out because I don't want to have problems with the transmission! (who does?). Here's some details on my car:

2000 Cavalier
2200 Engine
4-speed automatic
~49000 miles

Fortunately when I purchased this car used/GM certified I got a 75k warranty so if there is a major problem it shouldn't cost too much to fix.

I know the engine is about ready for transmission flush and fill, and I'm going to get that done very soon. Would that fix the problem, and/or how expensive is it?

Here's a summary of questions:
1. What might be causing the transmission to rev / jump?
2. Would flush / fill possibly fix the problem?
3. Is flush / fill and/or other minor transmission maintenance expensive?

Thanks in advance!