Help me out, please. Distributor...

[mhicks]

Ok, I have a '73 short stepside. I put a new crate motor in it about a year ago. At the same time I purchased a GM Hi performance electronic distributor and installed at the same time.
There are times when it will not start. Doesn't matter if the engine is cold, hot , humid outside or dry. I get in to start it and it just cranks and cranks, no start.
I have installed a new coil on top of the distrbutor.
I have replaced the distributor cap, I have ran a new wire to the distributor from the fuse box.
I have replaced the ignition module inside the distributor.
I have replaced the rotor cap.
I have swapped back and forth between the modules.
I have repaced the wires from the distributor cap to the inside of the distibutor to the module.
I am running out of things to try.
When it doesn't start, I can take off the distributor cap and play around with the wires, or touch the little spring loaded contact on the rotor cap, or the spring loaded button on the distributor cap. Put it all back together and 80% of the time it will fire right up. If it doesn't , I can go through that process again and bam, it fires right up.
Have any of you EVER ran across a problem like this?
Any help would be appreciated.
This has been going on for about a year now. It may not start twice a week, one a month or my go for a couple of months but it will always not start again. I am nearly 50 now and that distributor is way back there. Hard on an old man.