That would be a sign of a bad valve seal, oil leaks down into cylinder when cooling down. The smoke should be blueish, white smoke is water and unless your dumping gas in the pipe it shouldn't be that. Excess burning fuel is black smoke. I'd say it is from that seal, just enough oil is fouling the plug, but I wouldn't doubt if the sparks a bit weak also to foul that easily plus the hard starting, I've seen a plug that was a ball of sludge yet still the motor ran (smoked like hell though).
An Iradium should cure it as it is for now to get by.
Pull the plug and spin it over to have a quick look at the spark.
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