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Hello excessive build up of pressure in the radiator is usually caused by a blown head gasket or a cracked cylinder head.
This causes combustion pressures to overload the cooling system and symptoms such as you describe to occur.
You can test for this with a combustion gas leakage test kit available from your local parts store. It consists of a tube that attaches to the filler neck and inside you place a little of a special indicator fluid.
You start the car and generally on most types squeeze a rubber bulb on the tube to draw gases from the cooling system into the tube. If there is a blown head gasket or cracked head the fluid will change color. The kits cost about twenty dollars and are readily available - even through amazon or ebay.
The Rover v8 has been known to demonstrate the signs you have explained when a head gasket has failed or a cylinder sleeve has slipped. With the age of your vehicle I am going to lean towards a head gasket leak. Either way the truck should not be driven until proper diagnosis is executed. I recommend taking the truck to a mechanic and paying for 1 hour of labor time for a compression test... All cylinders should be 130 or higher and deviate less than 10 percent between cylinders. Keep us posted.