after finding a low milage engine and then spending a fortune rebuilding it ,the temp guage has started going from half to 3/4 then back to half in a matter of seconds or when it happens and i pull over and turn the car of if i turn start it again its back at half way ,just fitted new head gasket ,bolts ,radiator hoses fan clutch t stat thermostat housing the whole lot ,the car is running great loads of power and smooth since ive built and put the motor in,but when it goes to 3/4 and i pull over there is no pressure in the rad or anything like that doesnt smell hot,i got home today wipped the plugs out and did a quick compression test and all is fine ,the first 1200 miles on this motor were brilliant now this , its getting to the point were i darent drive the thing incase of overheating ,the temp sensors are also brand new ,this car is turning into a money pit ,,,,any help greatly appreciated thanks