Transmission problem
1999 GMC Sierra V8 Four Wheel Drive Automatic 143000 miles

Hi. My truck continually shifts up and down while accelerating until it hits about 65-70 mph. It does this in normal acceleration. If I "punch it" it doesn't seem to do it other than normal shifting through the gears. I had a friend of a friend drive it and listen for it. He seemed to think it was electronic not the transmission itself. Is there a control module or "computer" for the transmission? I don't want to have to dump a ton of money into a transmission. With gas prices dropping so much right now I'd like to keep it for a while longer. I love driving it and it has served me well as a work truck. Any ideas or things I can check?

Thanks
Doug Randall