Transmission problem.

Dodge grand caravan 1996, 150,000 M Transmission rebuilded 3 nonths ago, the problem came on only two weeks ago

It is allways the same. Driving over 45 miles My wipers go on one time just before the trannyl suddenly downshift to 2nd gear and does not go any higher. When I you pull over, put it in park, kill the engine, and then re-start it, it works fine for another two o three days. I did a computer scan and I got a transmission relay code, I don't even know where that relay is or if can be a problem in the circuit some where before or after the relay.
At least with the wiper thing I get a warning before happens so I can slow down