1992 F-150 4x4, 5L manual trans. 140k miles.

Lately it seems that whenever there's high humidity with rain/snow the truck
doesn't want to start.  The temperature itself doesn't seem to be the issue;
as long as the air is dry the truck starts fine.  The truck cranks fine, but
doesn't catch.  Occassionally, if I keep cranking, the truck *will* start,
but run rough for the first minute or so.  Before I start randomly replacing
ignition parts I'd like some sort of idea where to look first.  The plugs
and wires are a couple of years old, but the cap and rotor are new.  There
also seems to be a strong smell of gas when this happens, almost as if the
engine were flooded, but it's EFI, so that's not possible, is it?

Thanks for any suggestions.