Transmission stuck in third but only sometimes

I have a 1997 Blazer that just started getting stuck in third gear. But when I let it sit for a few minutes it goes back to normal. Also, the check engine light comes on when it start and goes out a few days later. I changed the fluid and it was clean. I think it might be overheating. I had heard some people talking about a coolant valve sticking. How can I tell? Any other ideas? Please help. Getting ready to seperated from the military and this is families only vehicle!