HELP PLEASE!

I have a 96 honda accord lx with 70,000 miles on it, well for some reason
whe ni accelerate my transmission will rev up about 2-3000 rpms before
shifting like its slipping. Its been doing this for awhile. I got a
transmission flush. And still having the same problem. But heres the wierd
part. After so long of driving it doesnt do it. It shifts fine. Whats the
cheapest way of getting this fixed? Also I dont have the engine light on.