[FC] weird electrical problem

[Wild8bill at aol.com]

I've been away for a couple weeks, and the rampside decided to stall out on  
my girlfriend while I was gone. she got a ride home, went to get it the next 
day  and it started and made it most of the way home (stalling at some lights 
until  refusing to start about a block from home). I just got done checking it 
out,  found that it was electrical problem. I tried to start it, dash lights 
came on,  but turned motor over 3-4 revolutions and the power goes out. I 
checked and  cleaned all the connections, and had power to dash again. Any large 
drain  (headlights, starter) causes it to lose power again, with disconnecting 
and  reconnecting the battery solving the problem, until I try to start it 
again.  Using my 12 volt tester, I found that the chassis actually becomes 
POSITIVE when  the power cuts out. disconnecting and reconnecting battery ground 
cable reverses  it so chassis is NEG. I don't know why it does this; bad battery? 
The weird  thing; the battery ground cable, that goes to the engine block but  
still attached to the NEG battery terminal, will be  POS, and touching the 
tester directly to the NEG terminal will make the tester  light go on! Before I 
go buy a bunch of parts I don't need; does anyone know if  I have a bad 
battery, bad cables, poor grounding, or some kind of gremlin in the  wiring harness? 
 (bypass already done on rear harness  connector)