why does my falcon fer boiler keep going off ?

my falcon fer boiler serial no.9946L60353(about 6 years old) keeps going off after about 2 minutes. It will come back on again after it has come out of thinking mode (red light). I have tried moving the thermerstats either side of the burner which seemed to work for a short time (it may just have been a fluke) but problem is now back. This does not seem to affect the hot water side. Everything mechanicaly is working fine it's as if something is telling it to switch off. Also why does it keep losing pressure, put some in and it is nearly at 4bar.