Whenever I have any sort of inconsistent electrical problem I start with the main engine ground. A good way to diagnose a bad engine ground is to use a set of jumper cables. Attach one black clamp to the negative battery post (double check!) and one black to a sturdy part of the engine - intake manifold is a good candidate. Leave the red clamps hanging free You are creating a temporary extra-strength engine ground. Make sure you have a good bite. Check to see if the problem goes away. If it does you can concentrate on cleaning up the ground connections -if not you can rule it out and trace the problem elsewhere.

Check your fuses. Pull each one out and brush the contacts. Replace the fuse and twist it a bit to get a good connection.

Examine battery connections at the post. Just pull them off, wire brush the terminals and reinstall the cable.

Some of the most useful tools in my toolbox are a $30 multimeter and a $1 dummy light electrical tester.

steve