Update on this: The skimmer had calmed down a bit to the point where the collection cup was about 3/4 full of essentially clean water every day. Not immediately overflowing but not skimming usefully either.
This morning, I had a few minutes so I pulled a few rocks from teh tank and was ripping hair algae from them when the skimmer suddenly overflowed.
Ok, now I have some correlation: algae seems to be setting it off. This would tend to make sense because my water is spotless.
Anyone else ever seen this ?
A buddy has offered to lend me a high power UV Sterilizer for a few weeks in the hope that may help.