Baxi 105 instant He problems?

I have the above boiler 4 years old and am looking for any advice as really hope i dont need to get a engineer out as might be very costly.

The problem (problems) are - the warm water wont work at all when i put the taps on the (tap) symbol flashes but the flame light doesent ( so i assume the pump is working) as can hear a slight noise too.
if i turn my control to central heating and water - the (Rad) symbol light up but again no flame light and i hear the pump again, though after 10 mins of leaving it on this setting the flame kicks in and my rads warm up and also during this i can also get hot water out of my taps( this is only way i can get hot water.
when all rads are full on the flame goes out and will not then come back on at all.
only way to get it back on is to wait till system is cooled right down and reset the boiler.

Any ideas folks??

+flame fault does show and also but not all the time the safty theermo light too