Sabrina,
I'm going out on the old limb here, but that isn't new. My uneducated guess would be the cooling system is plugged up somewhere else, and WITHOUT a t-stat, it can overcome this problem. With the thermostat installed, it can't over come the problem...so it lets you know!
My first thought would be plugged tubes in the radiator. You can peek in the fill hole and see the top header and tubes. If they look like grampas veins, you need help.
I don't think the block in these engines plug much, so the next item to check would, of course be the water pump.
Please don't tell me you have a new radiator and water pump...I've had a tough day!
Stuart
