One way of checking your radiator is to check for heat from top to bottom, first thing in the morning from cold, (If you can, remove fan cowling and fan), start engine and feeling top hose (being carefull of moving parts) as soon as thermostat opens (you will feel hot coolant through hose), let it run for a few more minutes and then shut eng off. If you can't remove fan, remove fan cowling and move it away from radiator, now being carefull not to burn yourself, feel the radiator from top to bottom,(It's a good Idea to remove the key first), If it doesn't feel uniform or cold in spots it's possibly blocked. The only other thing I can think of is maybe your water pump is cavitating due to worn pump vanes.