Hello all. I have a problem with my vaillant combi boiler. It has an intermitant fault with the hot water, the heating side appears to be ok. 50% of the time it works ok but sometimes when h/w has been used and then when it is called for soon after the fan and pump run (suggesting to me the diverter is ok) but the boiler takes ages to fire. I have fitted a new thermister which made a slight improvement. Sometimes when the boiler is not lighting a gentle tap on the front panel seams to get it going (this may be just a coincidence as sometimes it does not work) i suspect this may be a PCB fault but which one as there is 5. i have checked the pin which is pushed into the gas valve and the small bore pipes around it from the pump all seam clear as advised by vaillant. Also i think i need to lower the start gas pressure as it comes on around 3.5mbar then rises to to its max. Is it possible to get the boiler to run on the low gas rate so adjustment can be made ?