all gauges go flat while car still running

Ok, Over the past 6 months I noticed I'd be driving and glance down to check my speed, and all of my gauges are flat. Ex: I'd have a full tank of gas, but would be on E, I'd be driving but going 0mph, 0rpm, digital odometer blank, etc... I've kept up with all scheduled maintenance (2001 Xterra), I've taken it to the shop twice now for these specific issues, and they have yet to find the problem. Replaced battery terminals, fuel filters, transmission fluid, timing belt and it's still happening. No one can find an electrical short, either?! It's not an everyday thing. Gauges will go flat, and I’ll have no problem with the power, but the loss of power never happens unless the gauges are flat. All other electrical works, lights, wipers, stereo, etc...it's insane! Any help, thoughts, advise would be so, so appreciated!

2010, sometimes when I try to start the engine, the dash lights go out and apparently all electrical is dead. After a few seconds, I can start normally. However, the trip odometer is reset to 0. Any experience with this sort of behavior?

loose ground strap can do that.