for some time now whenever I first start my 620, hot or cold, the bike seems to be running on one cylinder for several seconds while I rev it a bit to keep it going, then catches and runs fine. Since the problem first appeared, I have changed the battery (it was 4 or 5 yrs old), changed the plugs, and run some STP fuel injection cleaner thru it. No apparent improvement. I'm thinking it may be getting worse, so I guess I need to start looking more closely at it.
Any thoughts?

while the engine is running, pull off one spark plug connector. If the idle doesnt change, then that cylinder is the one not firing. If the idle drops considerably or dies, then that cylinder is firing properly. After you find which cylinder is the culprit, you can test for spark at that particular cylinder when the problem occurs. If you dont have spark, check coil (ohm readings compared to book, or the other coil on the bike), check connectors, etc. If you have spark, listen to the inector with a stethoscope wand, or a long thin screw driver against the injector and the handle against your ear. Listen for the "tick tick tick...." which will tell you that the injector is working. 

well, yeah, the first thing I need to do is determine which cylinder is causing me a problem. The idle 'catches' maybe 3 seconds after the bike fires, so it doesn't give me much time for pulling off plug wires or listening to fuel injectors, tho I appreciate the feedback. I was more curious if anyone else had run into this effect, and what was the outcome. Would  a coil or regulator going bad hesitate for a few seconds before firing? Would a failing fuel injector take a few seconds to start functioning properly?

i got the same problem in my bike is weird though first it was the vertical piston an then the other one, i havent sit down and look to the problem yet but what i have noticed, when i took the spark cable off (spark failin) there is a electricity lik i see the spark comming out of one of the cables close to the selenoid and the cable had a litle openin( i dont know if is normal) but u can check ur wairin where the selenoids are( cause is behind the battery they bend too much and rip) if u see everything allrite then go to the ohm check, ill let u know if i fix it.

and is the same as yours after i accelerate it catches.

found the culprit: the spark plug wire on the horizontal cylinder had pulled out a bit (no doubt to its 'relocation') from the end cap on the plug, and wasn't making a good connection.

DOH!

Now it starts like it always has: cranks once, a small backfire, then off and running. I would love to replace those wires, but they don't appear to be removable from the coil end.