Possible torque converter problem?

I have a 98 4 door dually with a 7.4 liter and auto tranny. While towing a load uphill the transmission shifts down and it seems to slip for a while until locking up. It will barely maintain speed but not pick up. The tach will drop a few hundred rpms when it does lock up and the motor then starts pulling better and it will pick up speed and go on. This slip varies in length of time. Could this be a torque converter problem, some other problem, or is it just normal? Thanks.

It is possible that the TCC Regulator Valve is bad. See http://www.transmissionspecialty.com/parts/parts/34994-01K.htm

Someone would have to pull the valve body to be sure.

//2000CAYukon

does it do it after a long period of coasting with no throttle application, such as down a hill that leads right up the hill it slips on? my friend had a car do that and ended up needing a new tranny. then again, it was a mercury sable so thats probably a completely different design, peice of shiet passenger car versus heavy duty chevrolet power. i dunno lol but i would look into whatevers a cheaper fix before more expensive stuff... that is unless you have a older one lol with a turbo 400 that costs 200 bucks for a rebuilt one.