Hot water from potterton puma 80e just warm?

i have a potterton puma 80e 10yrs old. Heating appears fine but tends to fire up run for a minute or two and switch off over and over. System eventually gets hot. New heat exchanger fitted in last 12 months. When hot water tap turned on it flows fine but only gets hand hot. 'superburner' ignites and makes all the right noises etc etc ........ any ideas