Have you ran the engine without the radiator cap on? My truck was running fine with a blown HG, and the first clue for me was a round of broken hoses and heater core. I ran it without the radiator cap, and bubbles kept coming up through the radiator. It was also losing a small amount of coolant over time. Here is the source of failure on mine:



The metal part of the headgasket stretched then split right near a coolant passage, allowing the coolant system to be pressurized from the cylinder.

It sucked, but it also was definitely the right time to upgrade the cam! <img src="/forums/images/graemlins/smile.gif" alt="" />

Hopefully it's not the case with yours, have you replaced any hoses, water pump, etc. lately? Maybe you just have some air that needs to be purged from the system. You can do the park-on-a-hill-and-run-it-without-the-cap-test to purge the system of air. If you run it for a long time, and the bubbles don't stop, I would suspect headgasket.