Vokera boiler?

I have a compact 25 which keeps going to red on the led display and stops working. The boiler if it's not been in use for a day or so will run as normal then stop, I've had 2 gas engineers out to look at it but neither have been able to fix it. The condensate trap is free flowing, a couple of thermistors have been replaced and the PCB also has been swapped for a new one. The flu checked, pressure is good etc. We are all stumped by this but as it seems to work ok after a day or so it seems like something keeps getting choked then frees itself off over the time its not being used. Any ideas?