I've got a 94 Ford Ranger 4.0 V6. Recently after I changed the oil I popped the radiator cap to check the fluid level and topped it off. Now I don't know if it was a coincidence, but afterward anytime I drive the temp gauge goes nuts. You can tell when the thermostat initially opens, but shortly thereafter the gauge goes all the way up to hot and then immediately back down to cold. Its pretty consistent. Could I have an air bubble in the system or is my thermostat toast?