Okay, a guy in the Volvo newsgroup gave me a possible diagnosis. He said that the hose between the tank and the pump can crack, and will suck air if the in-tank pump is dead (as mine is.) That makes sense, as the car will idle a second or two while the non-aerated fuel downstream of the pump makes its way to the injectors.

Then, when the aerated fuel hits, it dies. This doesn't take but a second or two because excess fuel is constantly being routed back to the tank. It wouldn't take much aerated fuel to prevent the pressures needed to operate the injection system.

I'll be checking that out this week.