Transmission

I Have a 98 grand cherokee. When I am driving it and reach about 45-55mph it seems like the transmission is either down shifting back to fourth or into overdrive, then comes back to fifth. I can tell cause the rpms are around 1700 in fifth gear doing that speed and then they go up to 2000 or around there and then back to where they are supposed to be. I asked a couple of people they said sounds like a sensor but wouldn't tell me which one. It will do it for like a week or so then run normal and won't do it for a while then do it again. The longest it went was like a month not doing it. Please help me.