Halstead Ace Boiler Hot Water Problem?

Turn the hot water on - burner ignites water starts to get hot then burner goeas out after around 30 seconds - water goes luke warm/cold - continue to run the tap and after around 20 seconds burner comes on again water heats up then - yes yet again - burner goes out. It will do this continually for as long as you have the hot tap running!!!! Have seen a lot pointing at Diaphram possible secondry heat exchanger needing cleaning but any firm ideas as to what the problem is???????? - two young kids not a lot of spare cash to pay a plumber have the boiler manual and am pretty nifty with mechanics and stuff so any firm answer will be appreciated - should be easy enough using the manual to replace whatever componant needs replacing/cleaning!!!