Transmission problems

My truck has always had shifting problems. My dad bought the truck in 1995. He sold it to me in 2014 with 64,000 miles. He rarely drove it. About four years ago, had clutch and pressure plate replaced. Still was hard to shift into gear, but we made it work. Fast forward to about two months ago. It would not go into first unless you turned it off. Struggled through and it finally got better and started shifting better than ever. When the shifter was hanging up, it would stink like fluid burning. After a few days of driving, it started slipping in first, second and third. Fourth and fifth were fine. Today started slipping in fourth and fifth. The burning fluid smell was awful. Came home and changed fluid. No help, slipping worse. By the way, clutch is good, shifter is shifting correctly. Thoughts?