Move dash wires = no start

[HIGHWAY BY THE SEA]

Even though I finally have the 67 GMC 3500 running, I still have not completely fixed all the wiring issues. I have been fine-tooth combing the under dash wires, and the headlight and taillight ones as well since I bought it. I have fixed 99% of the wires that the squirrels had for lunch, and replaced any otherwise bad ones I have found. For the past several months I have been dealing with the sight of a mass of rainbow-spaghetti (wires) where the gauges will be installed soon.
But I have noticed an annoying problem ever since it has been running. If I move these dash wires, the truck (when the key is turned) mysteriously will turn over, but will not fire. The first time this happened, I moved the tangled mess back to where I had relocated it from and this fixed the issue. Whenever this happens, it also sometimes makes a the starter turn weakly, unless you mess with the ignition wires.
But today I started cleaning up the wiring mess by zip tying them together, and I must now figure out what the problem is, not the symptom. Any ideas from the electrical experts?