How can I fix my Potterton Puma 100?

I have a Potterton Puma 100 which keeps closing down.
I seem to cure the problem for a while but then it shuts down again.
Having been problem free for the past month it decided to stop on Sunday (very nippy). When the heating was turned up a clonk was heard from the boiler cupboard. The pilot light was out but the fan continued to run and run and run. When the pilot was relit, the boiler worked once more, but within a minute or so the boiler became noisy making a gugling noise. At this point the pilot light went out. Again the fan continued to run.
The pump is free and spinning, the fan is running freely, the preeure gauge indicates the pressure had been into the red though I cannot see how to reset the red pointer.
2 days ago I replaced the 'DHW Flow Micro Switch'.
This solved the problem. It fired-up normally with no untoward noises. The only noticable difference was the hot water pressure/flow. When I tested the hot water at the tap, water flowed quite slowly (as per normal). After 30 seconds or so the pressure/flow increased dramatically. The water was hot and all seemed fine until today when the boiler again made a clonk as the programmer was turned-up and again the pilot light went out.
Has anyone any ideas ?
Could it be the hot water flow valve or the wax module in the diverta valve ?
Gary