Potterton Suprima 50 cuts out soon after ignition of main flame?

I fixed the 'lock-out' problem last year by resoldering all the suspect joints on the PCB. After working OK for a few times at the beginning of this heating season, the boiler operation has become intermittent. The first heat request of the day is usually completed OK, but with later heat requests, after a normal ignition sequence, the main flame switches off after 20 to 30 seconds or a minute or so!
The LED indicator sequence is correct for a normal ignition, but when the flame has gone out the Green LED is flashing 2 per second and the Red LED is OFF! This would appear to me to indicate that the boiler thinks it is too hot (from installation the boiler temperature control has been set to about the 1 o'clock position & the boiler has always cycled on & off when the room temperature is approaching that requested. This I have taken to be normal), so could be caused by a faulty sensor. I would appreciate anyones thoughts on this problem.