Your '99 is fuel injected so it shouldn't have the clogged jet issues that would be likely in a carbed bike that sat around as much as yours apparently has. Though I wouldn't completely rule out some clogging in the injectors and/or other fuel related bits, it is fairly unlikely if the bike generally starts and runs fairly well. Check the obvious stuff, particularly battery condition. the voltage regulator/rectifiers aren't the best in these beasts and can both undercharge and overcharge, either way causing your battery an untimely death. Does it always crank over quickly but just not start every time? As for coils, yes they make a surprising difference. I put a set of Dyna coils on my 900SS SP, just 'cause it was the last mod on my very basic list (gearing, FCRs, pipes, seat, coils) and didn't really expect any significant difference. Well the difference was shocking - pun intended - with the bike springing to life much quicker at the merest flick of the starter button and definitely more low-end snap. It now does easy 2nd gear roll-on wheelies. A good mod, IMO. Anyhoo, give us a bit more to go on symptom-wise for your sometimes non-start issue and let's see if any other ideas pop up.