1994 Ford Majestic conversion van transmission issue

I have a 1994 e-150 ford conversion van with 125 000 miles
6 cylinder engine, E series transmission
I am trying to diagnose what seems to be a transmission problem.
80 % of the time the van runs fine but occasionally at about 45 to 60 miles per hour the tranny will kick/slam back and forth between gears and makes a winding noise then after about 30 seconds it is fine.
I am mondering if it might be a computer problem?
Could you plug it into the computer at a ford dealership to find out?
Its a great van but my wife is afraid to go on a trip until I figure this out.
Your help is appreciated.
Thanks