We're going to have to stop meeting like this! My son had a problem just like yours on his Turbo 300D. One of two things fixed it. He replaced the temp sensor and then found a hairline crack around the neck on the surge tank. Obviously the surge tank was leaking pressure and substantailly lowering the boliing point, probably causing the gage to jump around. Anyway, check around the neck of the tank. Hope that helps.