flexicom goes to F1 afterapprox 2 hours, every time

[bjorkasen]

flexicom 24cx goes to F1 error and stops after 1hr 45 to 2hrs 15 every time without fail. Probe was v hot so changed, but still same problem. Will not restart till round an hour has elapsed. At start of 2 hours, accepts demand for hot water but later in the 2 hours, demand for hot water initiates the F1. Changed probe and igniter unit and lead but has made no difference. Plumber plus his firm's national expert dont know what to try and get no useful help from Glowworm helpline who only read to them from the installation book. Its being done via a home emergency policy with limit of £1500 and concerned that they will use all this up by just replacing parts without having any idea of what it is. Have you met this problem before? Any help gratefully received and will be passed on to my plumber!! Many thanks from a very frustrated boiler owner!!