At an idle in park my 90 K5 warms up to 195 without overheating but when it warms up while driving it reaches well past the 260 mark and then shoots back down to the 190 range. It does this several times while driving on the expressway until it heats all the way up. Am I looking at a sticking thermostat? Fan clutch? perhapse it is because the truck has a 7 lb. radiator cap on it??? Anybody got any Ideas?