I had a similar experience & replacing the diaphragm & microswitch (cheaper than whole switch) on the diverter valve cured the fault.
You could also check the temperature sensor thermister on the hot water outlet pipe inside the boiler - sometimes these fail and sometimes the contact with the connector develops a high resistance - try pulling the connector on /off a couple of times.
Recommend that you get someone qualified to do these checks
Bill, February 2008
