No start when hot - low voltmeter, no headlights

O2 Blazer will not start when hot. The voltmeter shows low reading, but not completely dead,when this occurs. Instrument lights are on. Strong battery including adding a jump. Headlights will not come on but all other electrical seem to work strong. In past two days it has just stopped running whle driving at highway speed and second time when stopped for fuel it started and then died 5 seconds later. Starter does not engage or even click but will work if jumped at the starter solinoid. Once it even started and ran when jumped at the solinoid. New batteries, alternator, and computer brain box have not solved.

It sound like a (large) bad ground or connection somewhere. I think I would start with the headlites and work backwards since the car don't have to be on or hot. First test to see whether it is the ground or the pos. that is missing. It is probably the same open causing all problems. Turn on headlite switch and shaking wiggling a lot of stuff may also turn it up.