Looking for opinions on why my 1989 Chevy 305 isn't putting out very much heat. The temp gauge rises to ~ 180 then drops to ~ 150, {thermostat cycle} which takes some time. I put a temp gun on the radiator hoses and they seem to peek ~ 120. Not too hot to touch, like I think they should be. I put some cardboard in front of the radiator w/o any improvement. The heater is putting out ~ 50 degrees in the cab. Is it a possible clogged heater core? Too low of a thermostat? I've been inside the dash to if there was/is a possible problem with a diverter door, or something. ??