ice can do that as well.
But, chances are it is just bad spark plugs. Yes, NGK plugs are notorious for it. They can run fine at idle and crap when you give it power or vise-versa. There is no solution other than carry extra plugs.
I found the BR9EYA don't foul as often and have been running them instead of BR9ES. These plugs are identical except the EYA's have a groove in the anode to help prevent fouling.
If it isn't the spark plugs, then try this, change the plug wires from left to right and right to left. Don't worry, both plugs fire at the same time anyway, it will run fine. If the same cylinder drops, then you probably have a compression issue. If the opposite cylinder drops, then you know it is ignition.
Since the 2001 uses a single coil to fire both spark plugs, the next thing to look at is the plug wires. You can unscrew the the plug caps from the wire and then clip back about 1/8" and screw the cap back on. I usually use a little WD-40 or a dry lube to help out.
Be sure to inspect the cap for issues before you put it back on. The screw and the contacts are one piece. Meaning, they should move together. If they don't, they are broken. Which I've seen more than a few times.
2015 Skidoo Renegade BC 600 Etec
1996 Skidoo Touring E 380F (sister's)
1995 Skidoo Summit 583 LQ
1999 Arctic Cat Jag 340F
1999 Arctic Cat Z 370F (son's)
1985 Skidoo Tundra 250F
Currently working on:
1995 Summit 583 (friends)
1995 Summit 470 (friends)
1992 Polaris GT 440 (son's)