Jon,
First of all, get the sled to act-up, then pull one spark plug lead off at a time, then put it back on. The spark plug lead that makes the least difference on how it runs is the culprit cylinder. Check for spark...either by using a fancy clip-on tester or by putting a spare (good) plug on the end of the lead and check it.
If there's no spark, swap coils (see attached parts diagram) with another cylinder, then repeat the test. If the problem jumps to another cylinder, you have a bad coil (or ignition source to that spark plug lead). If the problems stays on the same cylinder, check wiring, etc. On that particular machine, I think the plugs (coils) might be fired sequencially....anyway, we'll figure that out when we get there.
If there is spark, check compression in all cylinders. The cylinder might be weak, and "limp" around, sometimes making enough power to fire, sometimes not.
Let us know how you make out !