PWMX 135 coughing like 60 yr old smoker

long story short.
bought a house, powermax 135. broke down 3 weeks later. I've quickly found the fault - fan's bearing (friction too great, unable to achieve certain revolutions). 135 quid. few months later - time switch. called a "specialist". after an hour he said, "will ring you in 2 days when find the part". didn't call me for a week, wife got ill (middle of the winter). I've dismantled it myself and found the reason - time switch. replaced it for £97. Fella called me few days later, "I've found the part, it's the motherboard. it will be £300 for the part plus labour".
Few months later - water pump. Easy, as it's standard (gold one). £50.

5 years trouble-free apart from the fact that 3 times out of 10 the boiler was coughing instead of starting properly and after 1-3 attempts of starting again was cutting the gas out (but the fan was still working, sometimes for 8-10 hours until I came back home/woke up)
so cold mornings during winters, crawling out of the bed just to switch it off and on again. Was bit annoyed by it so called another "specialist" and after £80 for a new electrode nothing got changed (surprise,surprise). So decided to show two fingers to all "specialist".
whatever.

Now after 5 years of resetting it in the middle of the night another winter started (2 wks ago) and the boiler developed a new fault - starts as normal (or not, 3 out of 10 times) but after 6-7 seconds of working properly the boiler cuts out the gas supply and switches itself off (the fan still works). then few seconds later it tries to start again, if successful - 7 seconds later switches itself off (good thing is that due to those 7 seconds sessions we're not freezing to death right now with our little baby).

any ideas what's that?

I'm desperate to buy a new boiler now but every "specialist" in the area (I'm from Lincolnshire) is so busy that is not even able to pick up the phone or reply to my voice message. Well two of them finally answered the phone and promised to pop in this afternoon but no one turned up (or ringed to say "sorry, too busy" or "fck off U bloody foreigner, not doing yr boiler"). Well, professional to the end.


any ideas what's cutting the gas supply? is it something related to the gas valve or air flow?
U see, as long as it was something related to electric fault or water, I could do that, but I've got no experience/license to deal with gas... but maybe you've seen it before so I could call another specialist and suggest him exactly what to do without replacing all parts  one by one?


Apologise for my wordy post, and for swearing..I feel so helpless...

thank you for listening
...