This morning my truck overheated while on the highway,I pulled over and it cooled off immediately(went from 240deg to 210deg in 20seconds)when idling then got hot again within a couple miles then cooled off as soon as I pulled over and idled(did this all the way home).upper radiator hose is cool aswell.yesterday the heater wouldn't get warm all day then is instantly got warm after 30 min driving...thermostat maybe?i put a heater core,radiator and waterpump in it last year...