1994 ford f150 conversion van problems:

I have a 1994 ford f150 conversion van that is giving me some problems. It acts like the transmission is low on fluid but shows
full. I can be driving down the interstate at 75 mph and suddenly it will slip out of gear and back in.
Sometimes it will do this for several miles and other times I can drive it for a month and no slipping.
I have had the ford dealer look at it and they say there is no problem with the transmission. It does this at slow speeds sometimes and even with the overdrive disengaged or engaged and the same with the cruise control.
Does anyone have any suggestions as to what to look for? I hate to go out for a new transmission only to have it be something else. If anyone has any ideas as to what to look for it sure would be appreciated very much.
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