CENTRAL HEATING PLAYING UP?

I have a chaffoteaux mx2 30 ff non-condensing boiler, hot water works fine, but boiler locks out on ignition for c/heating this does not happen all the time and seems to be worse when c/heating temp control set at half way, when the engineer came to have a look the boiler it worked fine, also i removed the casing to burner area to have alook and boiler worked fine on ignition until i replaced the casing, when the c'heating does work it will cut off at temp and then relight ok, Any advice would be appreciated thanks.