baxi 105e just cuts out intermitantly?

i have a baxi 105e combi boiler. it cuts out with the flame failure
light flashing .replaced both flame sensor & ignition sensor,the fault is still there if i reset it, it can last up to 2 days or 20 minuets? it will always reset . if it cuts out it sometimes takes 2-3 times to reset . when you reset it the fan starts up & the ignighter always sparks . have tested both micro switches all ok
poss. gas fault? im not sure but it might be when it switches from central heating to hot water? diverter fault? both plungers come out & hit the micro switches when it is working .there is no other fault anybody got any idears???