The temp gauge never gets past the halfway mark. I will have to check to see if it gets up to that mark at idle - my memory is failing me right now. I can check it tomorrow along with the hoses.

If it were a plugged heater core, wouldn't it just not work? When it starts working, it works very well - almost cooks me out of the truck. I also do not get an antifreeze smell nor is there any indication of a leak that I can see...

I can believe that the thermostat might be the culprit - I have no idea when it was serviced last so, I'll have to check into that...

Thanks for the ideas - I'll report back after I've had a chance to check things out tomorrow...