1994 Voyager loses speed
 

Hello everyone, I have a 1994 Plymouth Voyager with a 3.3L V6 engine with about 164000 miles on it. It has an automatic transmission.

I have recently been having problems with my vehicle. It seems to shift fine when I start out but, after getting to freeway speed about 65mph, it seems to shift down on it's own. When I try to speed up again I can hear the RPMs too high and sometimes it shifts and goes back down causing the van to pull .I also noticed it happens more in the morning or late at night and not really in the afternoon , more when the weather is cold. Since I work nights that is how I able to tell , when Im going to work (4pm) I don't get this problems but when I get out of work in the morning (5:30am)that is when it happens.
Would this be a transmission issue? Is there a sensor that needs replacing? Or would this be some other issue? Any help would be appreciated , thanks