If head gasket's leaking, you should be getting a little bit of water in your oil. Many shops have a device which can measure dissolved gas concentrations in your coolant an hopefully tell you if you have a gasket leak. Might also check to make sure the hoses aren't sucking in air anywhere and bleed the system a couple times. A couple air bubbles in the coolant lines can cause some weird problems. Also shutoff all accesory lines from the coolant system (water heater, air heater) try to get the engine as isolated as possible, this will make sure you don't have a leak in your water heater or somewhere else you don't expect. You can get a ballpark guess of what temp you're running at by just putting a meat thermometer in the radiator. Not a very exacting method, but it's a cheap way to find out if your temp gauge is way off.

good luck
david