1999 Plymouth Voyager Not going into overdrive

I travel a lot on the highways. During hot weather season, when I travel for several hours the transmission will drop out of overdrive. When I try to find a spot o pull over on the highway it will sometimes drop down to 2nd gear. After I pull over and turn the van off for about 15 minutes the van works as usual, as long as when I take off I go slow and smooth. I have also noticed that when I cruise between 30 and 40 mph that the van shakes a little. I have drained the transmission oil and put the proper transmission oil in for chrylser wehicles. Is it possible that a control module needs to be replace, and can I, a back yard mechanic replace it, or do I need to take it to the pros?