This problem is still not solved.
It happens only very rarely, on long highway runs (twice in the last six months, both times when I was driving over 65 for more than an hour).
There are some things that have been suggested here that I still need to check -- my apologies for coming back without having done full diagnostics. I've been busy with other stuff and the problem didn't recur, so I put it out of mind. However, I'd really like to soon take a long trip, and this is the one thing stopping me.
I was planning to replace my air filter (though it's not too old) to rule out the possibility of an intake restriction. However, I realized that because my crankcase is vented to the atmosphere, there wouldn't be any way for the valve in the puck to be closing due to intake restrictions. The vent is attached just to the crank and the valve cover, and then there's a tube which was going to a catch can -- but I wanted to rule out the possibility of a restriction in the can, and so it currently is just open.
So I figure I should next check for a vacuum leak.
Maybe I'll try replacing the oil separator -- I have a second somewhere.
And finally I guess I should do a compression test.
Anything else? This problem really sucks, as it's intermittent and yet serious enough that I won't take the van on any trips until it's resolved.
Thanks!