S-10 won't start

I have a 1996 Chevy S-10, 2.2 4 cylinder with 140,000 miles on it. The other day it stopped running. You can get it to crank, but it won't start. We checked the ignition module (10 times), and the coil packs and both are good. Changed the cam shaft sensor and crank shaft sensor, and put in new plugs. We have no spark at the plugs. Fuel pump works fine. The fuse box in the drivers compartment has a fuse marked crank that is dead on both sides so no power is coming to it.

After trying all we knew we took it to a mechanic who sid everything tested out good, so he decided to charge the battery and it started right up. He tested the alternater and charging system and everything worked fine. We changed the battery anyway and the cables and it started right up. A couple hours later it wouldn't start again. It would still crank but not connect.

It has to be a wiring problem somewhere, but what would start when you add the booster, start for about an hour, and then stop starting? The mechanic says all you can do is put an ohm meter on it and test all the wires. Any idea what wires to start with?

Thanks in advance for any info you can give me.