Ideal Isar continuing problem?

We have a problem with an Isar M30100 condensing combi - every time you turn on the hot water when the heating's on, it trips the boiler off with a L-F error (this happens many times a day). It also occasionally goes off at other times when just the c/h is on. We've had three different people round (2 from the manufacturer) and have had new ignition leads, a new pcb and a new ionisation detector, as well as having the condensate drain pipe checked several times, but it's still cutting out. They have now told us to "monitor it for a month", but that's a long time when you have no reliable heating or hot water! Can anyone help with a suggestion of what might be causing this? Thanks!