Hot water temperature varies?

Our heating and hot water system uses a Glowworm Ultracom 30cxi boiler. After servicing (2 weeks) ago the hot water coming from all taps varies between lukewarm and hot. (Before servicing is was fine)
Central heating is fine.
Boiler is set to max temp (62 deg C), I changed the hot water thermistor with no success.
Pressure is above 1.5 Bar.
Last week for ~2 days hot water temperature went back to hot, but now it is back to luke warm / hot.

Dave R
May 2010