[bamman]

When I got home tonight, it was at the 1/3 mark, and I let it idle a minute or two until it hit the halfway mark. It stayed there, so I turned on the defroster to force the fan on. The fan cycled on 5 seconds, off 5 seconds, repeatedly. That doesn't seem normal, either.

That would be normal if your A/C wasn't charged. The fan only turns on when the a/c clutch is engaged. The a/c clutch is only engaged if the low pressure switch is closed. The low pressure switch is only closed when there enough freon in the a/c system.