Had a 6620 from new.  I was meticulous about keeping the rad cleaned.  1000 hours on it and it started running hot.  Changed thermostats, fan belt and kept cleaning the rad.  Still ran hot.  Turned out it was the head gasket.  Not sure why the head gasket went it never over heated etc..  Make sure no bubbles in the rad when running.  All symptoms of head gasket if all else fails.  Are you having to add any coolant?

Whipper