I have a 2003 GMC Safari. It wouldn't start a while ago, so I have had the following replaced: spark plugs, wires, PCV valve, and the key cylinder. The fuel pressure is fine, (at least it was a month ago), the engine has spark, and starts sometimes, and doesn't sometimes. It hadn't started for a few days (I tried a few times a day), so I was going to have it towed to the mechanic because I am stumped. The tow truck driver ended up not taking it because he couldn't get in the driveway, but tried to start it and it wouldn't start. Later in the evening, I opened the door, and the door chime was sounding, the battery light was on, but the key was not in the ignition. I put the key in, and it started right up.

I am guessing one of a few things. 1. the computer has a short in it when it is damp outside (it rained a few days ago, and is still humid). 2. the ignition switch is bad, but only intermittently. Any other thoughts?

Three things common on your Safari intermittently not starting (1) fuel pump starting to go (2) ground to fuel pump making intermittent connections (3) ignition switch.

I agree ^^^

But being its intermittent, I'm leaning towards the ignition switch. Not the cylinder, but the electrical switch inside the column. They are prone to failures such as this in the Bravada'*, Blazers and S10'*.