Baxi Bermuda back boiler

I have a V4600 grey button valve type boiler which will not stay alight,it works for about 10 minutes, I have changed the thermocouple for a universal one (I couldn't find an original)this did not cure the problem so I got a new valve and fitted it, after about 2-3 hours the new valve is shutting down the flame just as if the thermocouple has cooled down and the grey button pops out, I,m now at a loss as to what to do, the thermocouple doesn't seem to be adjustable so I can't move it further into the pilot flame any ideas please?