Before you do anything else, have a good hard look at the battery. Strange things happen when the voltages get flaky. One of the most famous on our cars with the original batteries that were going out was the air bag light flashing a signal for low voltage WITH THE ENGINE RUNNING. Sometimes flaky batteries seem to test OK.
This sort of thing is a strong enough of a posibility that it may be worth trying to find a known good battery from another car to swap in for a day or two.
