Mystery Meat

Hi everybody, I just got my first Subaru, a 1991 Legacy L wagon AWD, and Im having troubles with the automatic transmission in it. I knew it had trans problems when I bought it, but the previous owner was not very mechanically inclined, and couldn't describe the problem very well. So, after I got it home, and checked it all out, I have come up with this so far: PO had the trans fluid changed, and fluid looks brand new. Trans fluid was a wee bit over full when checked correctly. (operating temp, shifted through gears, level ground) The car will shift through all the gears with no hesitation, including overdrive (and hold them). But it will only stay in gear up to an average 2500rpm. Above that, rpms sky rocket like it was shifted into neutral. It will shift into the next gear when rpms come back down. I say average, because when the trans is cold, it will hold a gear nearly to 3000rpm, and when hot, it only holds to a little above 2000rpm. Hoping to avoid a full rebuild for now. Any ideas? Thank you for your help.