I would say it's more likely to be scale and corrosion in the cooling
system;

The rad cap just keeps a certain pressure of water in, and the thermostat
stops the circulation of water until it reaches a certain temperature.

If you really want a new thrmostat and cap then by all means, they're
pretty cheap and the thermostat is the only real bastard to try to place.

Steve from dsn.co.uk stocks them and he's a helpful sort of a chap, but I'd
still go out and buy some sort of noxious flushing solution to try to
disolve all of the gunk in your system.

splam