You might have answered your own question.
"Like dead spots in the ignition switch."
Does this mean you get to take yourself out for Steak and Beer?
Next time it does not work wiggle the key around in the Start position.
It is a $30 part. I am talking about the switch not the lock tumbler.

Mine gave me problems with the AC interior fan once in a while. Fan would stop out of the blue and key wiggle fixed it temporaraly. Once replaced no more problems with the fan at least. Yours might be giving you intermintent start and no Horn.

Scroll half way through this post to see how the switch is connected to the back of the lock assembly. http://www.dieselgiant.com/repairignitionlock.htm
Unfortunatly I found this post after I had already removed the assembly.

You already seemed to check the ground out pretty well.
I chased the same starter symtom in a Jag XJS for a year before I found the battery to chassis conection needed cleaning on the chassis end in the trunk.
The fuse on the OVP probrobly got blown by a jump start.
You already found how it causes a problem in the Tach but I never heard of it going away once things warmed up.