[Quicksilver]

Ewww. Now that sucks. Nothing worse than a car that "might work".

Putting a temporary fuel pressure gauge on the injector rail can help rule that out.
A really cheap oscilloscope can test the reference sensors to be sure that they are giving a consistent 2.5V signal.
Go through all the electrical connectors in the engine compartment to be sure they are firmly connected.
But for a weird intermittent issue I'd think about pulling out the DME, opening it, and inspecting the solder joints on the circuit boards looking for any joints that aren't still smooth and shiny.

Other than that: You are going to have wait until it REALLY breaks.