Only thing that I can offer is that we have a 28i boiler, loft mounted over the bathroom. The thermostatic mixer delivers the shower but we rarely have to use the cold feed more than a tiny amount. We get the same as you, shower runs lovely then goes cold before heating up again. Performs much better when the heating is on at the same time so winter showers are fine (generally).
I changed the shower head to a cheap 1 spray pattern type and it improved things a lot, I suspect that there was back pressure from the shower head and that was leading to overheating and hot water cut off .... perhaps?
Tony, January 2015
