It may be the gauge as has been suggested. Try a new one. However if it is not, then radiator with not enough rows, such as from a six cylinder would not have enough cooling. Also warped heads will also cause heating. Had that happen before on a slant six with a new radiator and thermostat and such. This is not common but have known the water impellar inside the water pump to slip and not turn at the proper speed hence the anti freeze not traveling fast enough through the engine and radiator. Have seen a pump that did that. Wierd! Collapsed or crimped radiator hoses ( had that happen too ). When cold the hose looked normal but when hot it would collapse causing the anti freeze to stop circulating. Bad fan clutch if you have one. Had that MORE than once. Had defective thermostats. Running too lean can do this. Baldy is right. I had a Chevy 230 six jump timing on my 68 Chevelle. The distributor clamp bolt was loose, and the exhaust manifold got "Cherry Red" then would not run from retarded timing. Had to let it cool before fixing it. Can burn out your valves that way rather quickly. It happened back east at -14* in the winter on my way to my brothers place. I reset the timing and it was fine after that. No thermostat ( I know you have one ), can cause it to overheat because the coolant can go through the radiator too fast and not get cooled enough before its back into the engine again. Known of that before. Wrong heat range on the plugs can cause overheating but not usually but worth a look see. You may have too hot a spark plug. Go to a cooler one. After nearly working on rigs on and off for about 41 years you run into or seen most everything. I would get some Redline stuff called Water Wetter. Its irridescent red in color. Get about 2 bottles. Prestone makes basically the same stuff. Its bright green in color. It helps your antifreeze cool faster as it goes through the radiator. I used it here in southern Oregon where the summers get hot. 90's too over 100*. Works great! May need a third bottle if you have a 4 row radiator. Not a gimmick. I have it in both my rigs and it works great. One last resort. They do make a high performance aluminum water pump. It may be worth the extra $$ to get one. They work and are recommended for high performance engines for max cooling. About 30% more effective. 180 is the best for non computerized rigs. Check radiator circulation. If it moves slowly across the top of the radiator then have it fixed or replaced. I hope this info helps. Henry
