Trans issue.. Pls help
 

So im having a problem that SEEMS like the trans is slipping. Slipping is the best way I can describe it but I dont think its actually slipping.. It happens under light acceleration and seemingly only in 3rd and 4th.. No codes..

So for example say im going 45 and start speeding up real nice and slow.. Itll rev up about 1000 rpm more than it should, then drop back down to what it should be at, then back up 1000, back down over and over.. "What it should be at" is another hard to describe thing..

I just had this trans rebuilt (full rebuild) maybe 15-20k miles ago and I dont beat the hell out of it.

Fluid is a bit darker than it was a few weeks ago and a mechanic said it seems "sticky" but I dont feel it being sticky..

Mechanic says possibly the torque converter clutch.. Now I know very little about automatic transmissions so what do you think.. Any input would be very appreciated seeing as I already am uncomfortable with this shop and feel like I may have been "ripped off" for lack of a better term.. I mean, when should a trans fail after just being rebuilt??

Im bringing it in for an inspection as its under warranty and just want to know if theres anything I should prepare for..

Thanks much..