Rover 75 overheating

My Rover 75 overheated after 3 miles spurting out coolant from the radiator header tank. After I let it cool I picked out a rubber ring from the tank and matched it to the radiator cap. I replaced the radiator cap thinking that the coolant was blowing out because it wasn't pressurised. On the next drive the same thing happened after 7 miles. I replaced the thermostat. After a midnight finish at work I took the car for a ride out. All was fine after 12 miles. Engine turned off and a quick can of beer on a roadside bench like a wino. I drove the 12 miles home and reversed onto my drive. All ok. Until I turned off the engine. It seemed like a different kind of overheating. The steam and smoke was just rising from my engine bay. I could smell the smoke so I didn't lift the bonnet. The next day, the radiator took half a gallon of coolant. I'm already on my second engine at 70,00 miles. It seems that the exhaust gasses are getting into the coolant. Somebody recommended K Seal. Am I wasting my money trying to keep and immaculate 75 on the road?