I have a 1995 P38 Range Rover 4.6. It misfires when it is damp or wet and the electrics (headlights/wipers) go off for 5 secs and then come on again. Not much fun when you are driving at night!
Check all the earth connections and connectors on the BECM. Download RAVE for details of which connectors are relevant. Also check the battery connections are good and clean.