The van, again
Well, I drove the van back home from the mechanic's, a day and a half later (and $300 poorer) than when I started. The guy who worked on it, who sounded authoritative enough, told me that the problem was in a faulty ignition module that prevented the engine's spark plugs from firing correctly. I got home without stalling, so I considered that I good start.
I guess I'll keep my eyes on it for a few days and see how well it does. Can't say I'm entirely comfortable yet, however.