Sounds like a throttle trim issue. If you are letting off the throttle and it dies, then it sounds like the idle is set too low and or the throttl trim needs to be turned up.

As far as the glowplug, it is always suggested, but not always necessary, to replace the glowplug after the break in period. As far as a "bad" glowplug, usually it either work or it doesnt.If you are able to start the engine, then the glow plug is strong enough and still intact.