Turbomax plus 828 locks out if demand to sudden?

Turbomax plus 828. Following replacement of burnt out fan. Lock out red light comes on after 2 minutes and 20 seconds into gas ignition. If i increrse the temperature setting gradually (turning temp setting knob about 30 degrees) it will eventually reach temperature and keep going. I can repeat the process to get the central heating temperature up to the correct temp. similar problem if I draw hot water. If I draw maximum demand it locks out but if i turn the water up less than half way I can get a constant flow of hot water. another symptom is that just prior to lock out the yellow light goes on and off a few times before it eventually gives up
I am told it is a problem with the thermistor. Is this the same thing as the NTC sensor?

Nigel
March 2011