We have had a Glow worm Swift flow 100 combi for 6 years. For the past 6 months the hot water in the shower has been cutting out after a minute then briefly returns only to cut out again, and so on. The flow does not change - just the temperature. We also have longstanding problems with loss of pressure but we just top the system up as required. Advice on the first problem would be gratefully recieved.