Really Weird Electrical Problems - Ideas?

Earth problem through and through! And I would almost say that it's a loose earth as well, suggested by the fact thatsomethings are working and then not working. Cable size will be playing a part as well if you have a faulty earth, it will struggle to get the full current down the lines and as such you. If I have read correctly.

Try measuring the voltage from the (+) terminal and various earth through out the car. Anymore then half a volt (I think) is where you will have a problem. But check every earth anyways......Probably don't need to tell you this, but have you checked for corrosion inside the battery clamps? and are they done upthight? I'm sorry that I asked that but it's quite a common mistake.

I have a mate who is a "mechanic" he didn't drive his hotrod for about 3 months because it refused to start, he searched high and low for the fault, he finally asked me to have a look at it for him, I gave the (+) clamp nut one turn and it started...goes to show it doesn't take much.