Big Problems.. Need help!

[greydan]

If this was my car, I would;
Check and proof both battery cables..sound and clean. Check for clean and tight ground connections to the body and block. Clean and check all connections to the selenoid.
Check the mount bolts on the selonoid to fender wall, make sure they're clean and tight. Unplug, inspect, then re-plug the wire harness from the selonoid...(dirty or wire pulled loose)
Proof the selonoid with a positive jumper wire to the battery.
Suspect your ignition switch at that point.
Aerostar syndrome.
Got one, been there.