ethosb

It has only happend on two occasions recently. The transmission does not upshift properly and the truck will not get over about 30 mph. Acts and sounds like a governor does. I can push the gas to the floor and the transmission seems to stall. If I let off the gas a little and then reapply the gas it will sometimes shift and gradually gain speed. I have had the transmission fluid and filter changed between both occurrences. The only commonality I can find is "Heat". The first time it happend I had been in Atlanta stop & go traffic for about 1.5 hours and it was towards the end of the drive that it happened. The second time was yesteday. Again it occurred near the end of my drive, but I had been sitting idle for about 45 minutes earlier (It was shut off for about 20 minutes after the 45 minute sitting idle). Someone told me it might be a selonoid sticking. The mechanics can't diagnose it because it does not do it when I take it to them.
Any thoughts?
'99 F-350 Superduty V10 4x4 dually long bed