F350 Transmission Cycling
2001 F350 (50k miles). I was heading out last night pulling my 5th wheel, I switched off my overdrive (which I always do when pulling below 50mph) and about 10 miles in to the trip the transmission starts to cycle between gears. When I switch the overdrive on, everything seems fine (no cycling), switch it off and it starts cycling again. No engine light on or anything flashing, everything appears normal except this rapid cycling of gears when the overdrive is off.
Any ideas what might be causing it?
Thanks in advance for your help,