More than likely a failing flyback or failing caps associated with the HV circuit... It's a pretty common type of failure in Mac CRT's from about '97 onwards. The 5xxx Powermacs, and iMac G3's are known for having flyback trouble even when they werent terribly old, and even my 17" graphite Studio Display has been occasionally snapping for a while now... not often, maybe once or twice in a fortnight if that.

From what I'm told bad caps can blow the transformer and when the transformer blows it also invariably takes out a few caps so you kinda have to replace the transformer and associated caps at the same time... I can't verify this however as at this point in time, I havent had a machine that had one fail and was important enough or rare enough to warrant me actually repairing on component level... simply swapped entire A-boards.

I dare say the studio display will be the first one I attempt a repair on when it finally goes for good.