How quickly I forget past lessons learned ....
I had not thought about the plug being a potential problem until Fastsky mentioned it.
But this story comes immediately to mind now that my memory has been jogged....
Back in my first year in RC after running perfectly for 3 months my engine started giving me trouble. It would fly for a while then die, and I thought that it was a tuning / overheating issue. So I fiddled with it for a few hours and then one of the knowledgable guys at the field offered to help me. After a little while of fidding with the engine he asked about the glow plug and after my reply he said OS#8 ... I argued and from that point on all he would say was OS#8 ... well I didn't have any OS#8 plugs and besides that I was stubborn since the cheap plug I was using had been flawless .... I tried a new plug of my own with the same results and asked him if he had any more ideas ... OS#8 was the reply and he held out a bubble pak with a new plug in it .... he said if it didn't fix it to give it back to him and if it did fix it and I wanted to keep it it was $5 .... I had no hopes of it working but once I installed it I flew for the rest of the day without another deadstick .... when I handed him the $5 he just smiled that I told you so smile.

Since then I have not tried to be cheap on the glow plugs and have had very few problems, I have worn out my fair share of plugs though and if a previously working engine starts to misbehave that is the second place I look after fuel line inspection.