Liquid151

So this maybe a little hard for me to explain. So I got a 93 by auto runner that after about 15 minutes of driving, all of a sudden it feels like the trans drops out of gear and just revs. If I manually shift it to 1st it gets stuck into 1st. Yet if I pull over let it sit for a minute or two and start it back up or will run an drive fine again. All the fluids are fine, so maybe a shift solenoid or am I looking at something worse?