W9GTR

General question.
After driving my Skyline for the first time in weeks the other day it suffered what sounded like misfire under full throttle. I phone my dealer who reckoned that one of the plugs had clogged up and that a good run was needed. I have done around six hundred miles since,with the symptom becoming rarer but still liable to rear it's head on occasion. I took it in for service yesterday and the fault refused to appear lending some plausibilty to the plug theory. However isn't 600 miles a tad long for this kind of fault to clear. The only change I have made to the car is running Millers OB in recent months. I wondered wether if left for a while undriven this would have any adverse effects on the engine.
Any advice welcome
Cheers
Steve