There was wiring issue with the X-Endstop.
It would intermittantly trip while the Y-Axis was moving.

At times, nowhere near where it should trip.

I was able to validate using M119.

Replaced the wire - and G29 is now working again.

Next step will be to figure out a better replacement wire - as I think that a properly crimped wire will last longer than my cobbled together one.  Just need to sort out what the original ends were.

G1 Z15 F4800

Is moving your Z to a 15mm height difference from the original home position.

What you need to be sure of though is that your host, and the slicer have the exact same settings in the start gcode.

The slicer will override the host start gcode since it is executed after the host gcode is executed.

Things are a bit slow around here lately.
Check your settings in RH and your slicer especially the stating g code settings  is suspect your problem may be there.
Auto bed leveling can be problematic but is convenient when it works.
Are you running  a G28 home before your  g 29 ? 

This link may help.
http://marlinfw.org/docs/gcode/G029-mbl.html
You should alway home before any other commands ether manually or automated.
Happy printing

I had been running:

M100
G21
G90
G28 X0 Y0
G1 Z15 F4800
G1 Y5 F4800
G1 X5 F4800
G29

In terms of that guide - so much has changed since the solidoodle came out....
How plausible is it to flash to a more current version of Marlin?
The Z-Probe on the press seems like it was pretty unique (in terms of deployment/retraction).

Not the busiest forum, I know....
Any suggestions?

Things are a bit slow around here lately.
Check your settings in RH and your slicer especially the stating g code settings  is suspect your problem may be there.
Auto bed leveling can be problematic but is convenient when it works.
Are you running  a G28 home before your  g 29 ? 

This link may help.
http://marlinfw.org/docs/gcode/G029-mbl.html
You should alway home before any other commands ether manually or automated.
Happy printing

I'm doubtful to think that it is the X-axis motor or wiring - as it works perfectly for every command other than G29.
I'm even able to get successful prints, so long as I carefully supervise the  print start and manully assist during the G29 command.

Got it out, printed a few things with success,  and then something went wrong.

First issue was a shorted thermistor, which I've gotten sorted.

The second issue seems to be an issue during the auto-leveling,
where it is only using the left ~75% of the bed, and then grinds on the X-axis when it goes to retract the Z-probe.

If I cleanly reboot the printer, connect with repitier host - all 3 axis home like I would expect, and I can jog all 3 axis with what I would expect.

If I babysit the print, and pull the head toward the center of the X-axis before it lays down the first layer - I can make it through the full print without issue.

I feel like I'm missing something....