Update -- last week (while it was raining frequently) I checked and cleaned all sensor connections, replaced the distributor cap and rotor. No change. Over the weekend it dried out and the car started working fine. I replaced the plugs (the gap had widened a lot) and coil (even though it bench tested fine) -- car continued to run good. Last night it rained all night but the car started just fine this morning. So if this result holds, I'm going to blame the coil.