I run my engines so that there is some smoke trail when the aircraft is in flight. This tells me I am not running the engine too lean. You said the glow plug still glowed red on your driver, yes a worn plug will still do that. Check the element in the plug if the coil looks at all deformed ( some coils tight and maybe one or two spread apart), if they are
the plug is either worn out or has had an overheat situation.