I drove the car home from work today and it behaved normally, then I went out to drive it again and it only had first gear and reverse (would not upshift), it wasn't slipping so I decided to drive a little more to see what would happen, and it got second gear back, then third. I parked it and let it sit for a while, then drove it again and it only had reverse/first/second, then starred going into third now is completely back to normal.

Any idea what could cause this? I have had it ~3 years and changed the fluid twice in that time, and it isn't currently low on fluid. It has about 290k miles (broken odometer so not sure exactly), but has never had any transmission issued before now. I have no idea if the transmission was ever rebuilt, I bought the car cheap from someone who didn't know anything about it so I really have no history on it at all.