F150online Forums - Shifting in and our of 3rd gear

Revise(Shifting In and out of 3rd gear)
Just purchased a car off of someone about a week ago, 1994 Ford f150. The first 2 times I drove the car the tranny was fine. But I recently drove it around and the car is not shifting into third gear I can feel it shift into the gear but it instantly pops back down to 2nd. It also felt like it was having a hard time getting into 2nd but still works. I have been looking around online but can't find the same exact problem anywhere. And I cannot get above 30mph. This is in overdrive. When I put the car not into overdrive I can't feel the car switch to 3rd gear but it allows me to 40mph but has a very high rpm. I believe it is a bad solenoid but am not sure as I have seen people with a similar problem that do things anywhere from rebuilding or buying a new tranny to simply replacing a TPS sensor. If anyone has any clue on this problem a response would be greatly appreciated. Thanks!