Problem with Rotel RA 870

Question

After turning the power on the Rotel RA 870 amp the protection light stays on for hourse preventing me from playing my system. when it finally goes off i can play my music for and hour or 2 but it sounds under powered and eventually the light comes back on and these prevents me from playing music anymore. i have the amp for 10 years now and i only use pre amp section of it since i bought it. almost never switch it off so that may have something to do with the problem. PLEASE ADVICE