[twobjshelbys]

It can also be the water pump. When I first got mine it ran normal temp but took a long time to warm up. Later in the spring it started to warm up and then got very erratic - it would be immensely hot one minute and then go cold. At first I thought it was the temp sensor and air bubbles and even bought a Kirkham expansion tank to get the filler at the high point and an exit for air from the radiator. None of those things worked.

I eventually tracked it down through the process of elimination to the water pump - my mechanic and I couldn't see how but it was the only thing we hadn't checked. The impeller had a broken blade, and the internal push-on pin had been stripped. The impeller would line up over the exit to the water pump and impede flow. The radiator on these is so danged efficient that it was basically cooling with convection and until the ambient temp warmed up or the impeller blade just happened to land over the entrance, it would cool OK.

But for air, the geometry of the system doesn't really make it easy to bleed it. The exit from the water pump is higher than the radiator and the filler is either in the hose going to the top or in the top of the radiator. It makes it very difficult to bleed without some kind of expansion tank.

Another thing that really helps is to drill a bleeder hole in the top of the thermostat. It gives air captured at the top a place to go and also some minimal flow through the system with the thermostat closed. Without flow it depends on convection to open.

Let us know what you find as these troubleshooting hints are always useful to others.

Just another thing to check...