esands, long story short, the problem turned out to be the shortboard. More specifically, the volume pedal on the shortboard. My playing style depends heavily on volume pedal usage, for attack swells and rapid volume changes while accompanying, and frankly, the pedal on the shortboard just isn't made for heavy usage. I wound up buying a heavy-duty Boss expression pedal, and connecting it to the expression jack on the board, so now I use that for volume control, using the built-in shortboard pedal only for occasional wah or other effects. That eliminated the intermittent "sputtering" problem. This took forever to diagnose; after the main board replacement mentioned above, the problem reoccured, and Line 6 was nice enough to swap me a whole new amp - which behaved the same way, so finally we focused on the shortboard. It was deceiving because (as I wrote way, way above) at one time the problem occurred without the shortboard connected - I'm speculating this may have been a second, distinct issue which was corrected with the new main board, but the original issue with the shortboard remained. In any event, using the external volume pedal fixed the problem. Unfortunately, it introduced a new problem, in which the shortboard occasionally gets confused and loses connectivity to the external pedal (suddenly, mid-song, the green shortboard pedal LED lights up and we jump to full volume). I have to unplug and re-plug the expression cable to make it work again. This doesn't happen frequently, but often enough to be annoying. Honestly, after 5 years with my Spider, I'm pretty disappointed with it for live gig usage. It's light, portable, and versatile, and sounds good when it behaves, but just isn't reliable enough for onstage.