RCA F36650 36

Sounds like horizonal output transistor could be over heating. There could be other problems. But I would start there. You can find the part number on the schematic inside the set. If you change it yourself don't forget to use the included insulater. Part should cost between 15 & 20 dollars. You will need make & model # of set at electronic parts store.
Diagrahm inside set will show you location of transistor.