overheating?

Either:
(a). the radiator could be blocked;
or
(b). the thermostat could have failed (they will generally fail shut)
To check the radiator, have the engine running and warmed up.
Run your hand (or better still, someone else's hand) over the front of the radiator. It should be more or less the same temperature all over (and will be quite hot). If one side is cold, or the top is red hot and the bottom is stone cold, you have a blocked radiator. The bad news is that the only 100% cure is a new one.

To check the thermostat, have the engine running and at working temperature.
Check the radiator exactly the same way. If it's only lukewarm or cold, water isn't flowing and the 'stat will probably have failed. You may also hear the coolant boiling.
Again, the only cure is a new thermostat, but one of those is a lot cheaper than a radiator.

Also; DO NOT open the radiator cap after either test until the engine has cooled; unless you have made a reservation with your local A+E!

Phil
September 2008