2003 2500HD won't turn over...any ideas?

[VDZdmax]

I have been having erratic starts out of my 2500HD. Lights come on like normal and when you turn key after glow plugs go out nothing happens. No bump, no grind, nothing. All the dash lights go out like the batteries are dead, but let up on the key and everything comes back. Try it later, or again and it may start!? I have changed out the neutral safety switch, had a repair shop tell me it was the starter( it wasn't), and now they are stumped. I am wondering about the ignition switch, but don't like swapping parts just to "try" a fix. $$$ Batteries are fine. I thought I had seen a thread on this very problem, but can't seem to locate it. Was hoping someone a little more savvy on the searches could point me in the right direction...
Thanks