Auto Transmission shifting problem

Hi all,
I've got a 97 Disco I with a 4.0 engine and auto transmission. The problem I am having, or should I say, ONE of the problems I am having, is that when the truck warms up and I am travelling at about 40 mph the transmission downshifts to 3rd gear and to maintain 40 mph the engine rpm runs about 3000 rpm. Then it will shift back up to 4th after about 2 minutes. But will keep doing this off and on. When the truck is cold it shifts fine. I tried draining the tranny fluid and replacing with fresh fluid and a bottle of Lucas goop. I know that only draining the pan only removes about 5 qts but thought it would help just the same. But no difference. It almost sounds like a fluid temp sensor but according to the Rover people, it has one but it only turns on the overheat light, it has nothing to do with the shift commands. Any suggestions?
Thanks for the help :)
Bob
 

Hard to say, but I would suggest taking it to a tranny shop, not the dealer, and ask them, if you are lucky, maybe a tranny service will clear up the shifting issues.