I could be your starter motor?
If the brushes are worn and they have also worn the armature then you'll get arcing. This arcing robs the power out of your battery and lowers the health of the spark at the plugs. This means that your spark is dull/weak so your motor is trying to/firing on a weak yellow spark, not a fat blue one. Not sure if this can happen to skylines, but other motors it definitely happens, I've seen it and repaired it [well got the starter motor rebuilt].
If you crank the starter motor over and check the armature at the same time you'll see the arcing, they all arc a bit, but if its bright and strong then I'd get the starter motor checked.