My vokera Mynute 20e boiler keeps cutting out?

We had a new apartment built 5 years ago and have the original boiler which has had a few problems but the plumber who did the installation sorted out. It has recentlystarted to sometimes cut out and does not start up when the systems asks for heat, the red light comes on. We have a pressurised system, underfloor central heating system and a hot water storage tank. When I turn the reset knob it always starts and will work for several days or weeks. But just when I stop going to look at it or in the middle of the night it stops working again. A new heating engineer that came was not sure was wrong and said he would ask the help deask but that was 2 weeks ago so I guess I have to find someone else. I would like to avoid being ripped off if possible.