I had this exact problem a couple of years ago. There was an accumulation of dry fluff etc in the fan casing and the exhaust where hot air is blown to the drum. The restriction in airflow that this caused meant that the protection thermostat in the heating element was cutting out as you describe.
The solution was to split the fan housing (this is the aluminium casing at the top right rear as you face the machine) and clean out all the debris.
Once you've done this, you should also check that fan actually runs when it's called to.
Assuming a clean airway and a running fan, the air should now be heated to a lower temperature and the thermostat won't cut out.
If this doesn't work, my next steps would be to a) check that the water valve is firing coolant to condense the water vapour produced in the drum b) try a new heater/stat.
Hope the above helps you
Alphin White, May 2008
