I'd consider lopping 5mm off the end of the HT leads then screwing the spark plug caps back on really tightly for starters.
There are several things it might be, but you've just gotta go through a process of elimination.
I'm with Darrsi, HT leads cost nothing to do, and, if it isn't them that's causing the trouble you have just stopped it being a problem a way down the line.
I did mine because i noticed it running just a little lumpy for the first 10 or 15 seconds after first starting and I noticed one downpipe was markedly cooler than the other 3, so, 5 or 6 mm off each lead HT caps screwed back on (How Hard to get them started to screw back on) and the problem was solved.
Other than that I have no suggestions, as I have not had any other running issues apart from multi connector boxes, but that is another story.
Good Luck