2000 Ford Expedition Car will not start unless I disconnect

I changed out the ignition switch on my 2000 Ford Expedition over a month ago and everything was working fine. Then one day I got into the car and my anti-theft light, abs, and oil light was on and the key was not in the ignition. I tried to start the engine and could not. The engine would spin but that's it. We towed the car to the mechanic that night and the next day the mechanic said the car started fine and couldn’t find any problems. They hooked it up to the computer and only a theft code came up.

The car worked fine four about three weeks until yesterday when I attempted to start the car but it wouldn’t start. I disconnected the battery, and then reconnected it and the car started fine.

This morning I get in the car again and notice that my odometer numbers read "---------" but my key isn't in the ignition. I try to start the car and the engine spins but will not start. I disconnect the battery and reconnect and the car starts fine...Any suggestions? This seems to happen on rainy damp days.

Thanks,
Chris