Baxi 105e flame failure?

I have a problem where the burner ignites and after a few seconds the flames decrease the ignitor sparks again but within 2 secs it cuts out and flame failure light shows.
I can nurse it to work by setting the water pre heat up and when it starts run the hot water for 5 mins to keep it going, the heating system can then be left set on low and it will keep working. (had it working all week like that on low setting) If it is switched off for long enough to completely cool down ie. by powering down or the timer. The boiler will exhibit the same prob with flame failure but I can get it going with my pre-heat and run the hot water trick again.

Any ideas would be appreciated.