Can't find a similar question in the posting, so here goes!
My potterton performa 28 doesn't regulate the CHW temperature very well. I have replaced the thermistor and the behaviour has changed from cycling low to >80degC and (I think) dropping out on the safety thermostat.
Now, at low temp settings the gas comes on at low and after 10 secs or so modulates up over 5 or 6 secs and then reaches a simmering sound then turns off. This repeats all the time.
At much higher temps the result is much the same except that the gas modulates back down instead of turning off and will stay low for quite some time. Any ideas?