302 efi rough idle and kills

I have an '88 e-150conversion van w/302ci fuel injected engine. It just started to idle rough and kills at a stop occasionally. what could cause this? I do hear a pinging noise from one of the rocker arms. It has had some electrical probs before (door locks possesed when it rains hard) and the check engine light only comes on while driving, sometimes it'll stay on for awhile and other times it'll flash then shut off. Each time I can't figure out the code that it flashes cause I'm paying attention to the road. If I were to take it to a parts house could they find out what code it's sending even if it's not flashing at the time? Any ideas are welcome.

thankx Sean