Auto Tranny Problems

[FORSFED]

I put this here since it isn't really DSM Tech. Anyway, I found a tranny for my friends Stealth and I just put it in this weekend and there seems to be a problem with it. I can put it in reverse and it engages and the car will back up just fine. Drop it in nuetral and it responds. Put it in drive and it engages, however when you step on the gas it is VERY sluggish. It almost feels as if it is starting in second gear possibly... I took it down the road and stomped on it and noticed it doesn't downshift and you get up to about 50 mph where it should shift into overdrive and it won't (overdrive is not turned off). I checked the fluid, it is full and it came out clear when I emptied a small amount out. Jacked the back of the car up and the rear brakes don't seem to be hanging up, so I am stumped. I don't know auto's very well, so could it be a bad tranny? Are there any adjustments I can make? Anyone have issues like this? Please let me know, thanks.