I have the same with my potterton puma 100 i changed the thermocouple,no luck.got combi man out to look at it,hecould not find anythink wrong he turned the pump from 2 to 3 (full) and charged me £84 .I replaced the pump 2/3 years ago but not with a potterton one its a single speed one and twice the price of a three speed made by same maker i ran new pump on 2 it made heating quiter.To soon to tell if it worked i will let you know.
chris
February 2006