As a wild guess, the xenon tube in your light may be reaching the end of its life, but of course it could be something else altogether.
The company that makes the Snap-On units makes a whole line of timing lights, from inexpensive bare-bones to expensive fancy, and they're less expensive without the Snap-On name. I got the plain version. They're really nice units that work at very high engine RPM by skipping flashes as needed, without losing accuracy. Here's a link to a thread I started on the triples board:
http://www.kawi2strokes.com/forum/viewtopic.php?f=25&t=3619Many or most cheap timing lights have trouble at high RPM, displaying jitter and error, and this light solves the problem. Guzzis aren't as demanding on a timing light at a 3 cylinder 2-stroke, but it's still nice to be able to check the timing over the advance curve without being concerned about accuracy of the light.