My folks aquired a 95 or so Ranger, 6cyl auto.  The transmission likes to down shift itself for absolutely no reason.  It's been in a local shop but they cannot seem to get anywhere with it.  
The shop pulled the trans and went through it but it had no impact.  They replaced I think the MAP sensor and I replaced the TPS.  

Thoughts?

First one that came to mind is "They dropped the transmission?"  I hope you mean the pan and looked for metal particles, replaced the filter and filled it up with clean fluid instead.

 

The truck was having the problem, the transmission was removed and gone through replacing the usual warn parts.

Afterwards the truck did the same thing.  The pan was pulled and more was done, however I don't know precisely what was done.

A few trips later and a few electronic parts it still has the same issue.   They largely grew tired of putting money in it and I borrowed it for a bit.

Speed isn't apparently an issue, it has done it from 20 mph to 75 mph.  Not only is it
odd, its random.   May not do it for days, then do it several items times in a few miles.

CHECK FOR DIRTY LOOSE GROUNDS AND DAMAGE TO THE SHIFTER CABLE
AFTER THAT THE CAR MUST BE SCANNED TO DETERMINE IF THERE IS ANY COMPUTER ERRORS AND THEN ENGINE PERFORMANCE ISSUE'S