1997 Suburban transmission rumble

My Suburban 1500 4x4 has 150,000 miles on the 350 engine and 90,000 on the transmission. The truck runs great for 20 to 25 minutes then begins to have a rumble from the transmission when subjected to any strain in third or forth gear. The rumble occurs just when the torque converter goes into lock-up mode during acceleration, like going uphill. With a light touch to the accelerator I can feel the torque converter engaging and disengaging as I watch the tac rev up and down 300 rpms The colder the outside temperature the less frequent the problem. I have had the torque converter and the computer replaced but the problem is still there. The dealer is now saying the engine needs a valve job, cost $1100. Any suggestions?