van's completely dead now

This is a 96 chevy express van with a 350 and 145k.

I posted a couple weeks ago about my van having multiple cylinder
misfires. That problem sort of went away on it's own. Last night, it
died completely after some rain. It alternates between acting like a
dead battery and having no power at all. In dead battery mode, the
electricals seem to work, but you get solenoid click when trying to
start. When you try this, most times it goes completely dead after a
second or two of clicks. If you wait a minute, you can get the
electricals to pop back on. They sort of cut in and out. But there's
never enough power to start the car. And when it's dead, nothing works
like when the battery is disconnected. It has a brand new battery and
alternator that had been working perfectly. I haven't tried to
diagnose anything yet. Any ideas on where to look? My gut is telling
me it's the grounding strap.

brian