Potterton Puma combi boiler heating not working?

I have a Potterton Puma combi boiler that is now 12 years old. I know I should buy a new one, but cant afford it at all. The heating comes on and then goes off after 1 min to 30 seconds sometimes. We have changed the overheat stat, thermo couple, d.h.w/c.h temp sensor, checked the circuit board for dry solders but nothing. Now after changing these things the pilot light goes out when the heating has been put back on and takes a long time to light. The water works fine, but now I am left with no heating. Please help