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Mystery or a miracle or both

I have become the proud owner of a 1991 Ford F-150 long wheel base with a camper shell. It has a 300ci straight 6 with an overdrive transmission. When I first started driving the truck over a year ago it had about 45000 miles on it. Obviously it had not been used much. I have since added about 15000 miles so now I am just over 60000. When I first started driving the truck occasionally it would jerk really hard when shifting. It didn’t seem to matter which gear it was going into. The more I drove the worse the problem got to the point where it would drop completely out of gear while going down the highway. Now another detail. The jerking was much worse when the outside temperature was above 50 degrees. During the winter the problem was all but none existent. I thought I was at the least looking at a transmission rebuild. Well about two months ago I decided to try something radical. When I drove in town I would turn the overdrive off and manually shift through the gears. Since my business is not far from my home and I seldom get over 50mph I would leave the overdrive off. It never would get over 2000 rpm’s. After about two weeks of this I tried putting it back in drive while in town. Now there is no jerking or dropping out of gear. Just wondering if anyone has a guess as to why this seems to have fixed the problem?

I think I would have the fluid and filter changed in the transmission.

Sounds like you have goobers circulating around the tranny and manually shifting has resettled them in a better place. Fluid and filter change - yes; force flush - no.

Have you checked fluid level and condition?
Change every 30k miles.

I second changing the fluid and filter and will add that you should remove the inspection plate at the bottom of the bellhousing, turn the engine slowly and see if there is a drain plug for the torque converter. If there is, drain it as well.

Since you no longer have a problem, are you going to try to fix it?

Talked to a friend who said the valves may be sticking. So I am going to have the fluid changed and a new filter put in and go from there. Everything you guys said he agreed. Thanks!