Thanks for the responses. More detail: the problem cuts in and out as suddenly as a switch, but from several minutes up to an hour apart. Doesn't seem like fuel starvation from fuel line issues, that would be present more or less all the time at WOT. There are two tanks with separate primer bulb for each, then into common filter. Problem is present on either tank. Both primer bulbs firm up without problem. Carbs were overhauled summer of 2012, ran all last season without issue. The only fuel issue I can think of that fits the circumstances is some crud blocking a high speed jet, then being dislodged, then more crud causing the problem to repeat. But I have that big cartridge filter, it's changed yearly and of course I changed it right away when the problem began. There is a filter screen in the fuel pump but that was perfectly clean both times I looked at it. Bow tank was run down to about 20% level for winter and stabilised with Sea Foam. Stern tank was emptied completely. Fuel lines in hull were removed and drained whilst some hull fibreglass work was doen over winter. I did the spring runup with more Sea Foam added to bow tank and ran it for 20+ minutes while it coughed up all that gunk that two strokes seem to produce. Then filled the tank with regular 50:1 and all ran well for two tanks' worth before this issue arose.
I'll run through the shop manual carb diagnostics more carefully, see if I can discern some problem there. One thing that does make me wonder is whenever the engine is tilted forward into trailer mode, some fuel dumps into the splash well. That doesn't seem like correct function. If anyone sees something in the additional details I've provided, please let me know.