[monza]

I had exactly the same problems you are describing , drove me nuts!

Finally traced it down to the fuse box and the connection between the firewall side and the dash side specifically the ignition wire connection in the fuse box. At some point this connection became loose and looked like it was electrical arching between the two sides causing the plastic to become brittle and loosing a solid connection... I assumed continually getting worse and worse.
I bypassed the fuse box with that particular wire and it was fixed.

The actual cause was created by the bolt that connects the firewall side and interior side of the fuse box, at some point it looks like it was connected cross threaded, or a bad thread from day one? It seemed tight but it was not, it was cross threaded and could not tighten anymore (the T-nut embedded in the plastic was spinning) although from all indications it was tight.

Might be something to look at hopefully it helps??? Next time it happens go giggle the fuse box.... and try and start it, or have someone giggle the fuse box while you start it?