That's your Torque Converter clutch slipping. Time to save and save quickly! It's also time to research, and call around to find the best Transmission Vendor for YOU! There are a few good ones out there.
Goerend Brothers,
DTT
ATS
Suncoast
Do searches on all of them... . call around... . I can tell you what I know about 2 of them... send me a PM for that.
Josh