What causes high radiator pressure?

"High radiator pressure" - what does that mean? You take the cap off and it has built up pressure? The cooling system should have about 13 psi of pressure in it -- because the radiator cap releases pressure at 13 psi. Perhaps replace your radiator cap first - it's inexpensive. Ensure a 13 psi cap (factory recommended) is installed. The only way there can be way too much pressure is if the exhaust of the engine is back-feeding into the cooling system, such as with an internal engine coolant and exhaust gas intermix leak. This would be if there was a cracked engine block, cylinder head or breached cylinder head gasket. You would have other symptoms more than likely if this had happened. I don't know that your starting issue is due to a "over pressurized" cooling system either. Hands on testing for fuel delivery, air intake, emissions controls, exhaust flow and ignition spark are needed. Good luck!