Vokera Linear 28 - hot water ignition problems?

Hi
Have a linear 28 (2002) usually works fine, occassionally change a part or two, but at present am stumped with problem with hot water. Water won't get hot at all. Central heating seems fine. When you look at the flame, it comes on for 4 seconds, then off for 4 seconds repeatedly - never really heats up. Called round engineer but seemed to be working alright, got it to heat to over 41 degrees by finding the temp switch had broken off inside, so turned the stat up manually to just below max and then had good temp (59) - also changed the thermistor on the HW. Thought all ok but by evening the flame was doing the on, off thing again. In morning had a 01 code, reset, and checked all connection wires, tightened all connections possible and seemed to be working. By evening it had the same on, off, on, off problem. Next day checked the ignition electrode. Gave it a clean and adjusted gap to 3mm (it was about 4mm) and started all ok. Checked later in the day and all fine. Come evening back to usual ignite, out, ignite, out every 4 seconds and am now flumuxed. Any help really appreciated - just as an aside, the heating is set to come on early morning for 4 hours and early evening for 4 hours - don't know if that is relevent?
Thxs
Simon