It does sound a bit like a torque convertor issue. Problem is, I believe the torque convertor locks or unlocks due to the flow of fluid... I know there's solenoids but I'm not sure there's one that controls the lock up of the torque convertor.

If you want a cheap solution, go to your local pick and pull, find the same year (99 or 00) and pull the shift solenoids off the transmission. All you have to do is drop the pan and they are right there on the valve body, just one 10mm bolt and a wire clip for each, should be 3 total I believe. Should not cost more than $10 or so per solenoid. Put them on your rig and see if that fixes it. Way cheaper than handing your wallet to a transmission shop and not hard to do, just messy. Also, make sure your fluid is fresh and clear.