Mini Kossel - G28 Homing Problem

I have a problem that I haven't been able to figure out. When I send "G28" to my Mini Kossel, then the three carriages should all move up to the end stop. However one of the carriages (driven by the y stepper) moves and then vibrates a little ways away from the end stop. If I send G28 a second time, then that stepper moves in the opposite direction!

Here is a video:
http://youtu.be/USk278MbVGc

Printer: Mini Kossel
Electronics: Ramps 1.4 / Arduino
Firmware: Marlin

I have tried:
swapping out the stepper motor for a different motor
twisting the wires of the stepper and of the end stop
double checking that the wires between the stepper motor and Ramps are connected in the correct order
tuning the stepper drivers
swapping out the stepper drivers

Attached is my Configuration.h file.

Here is the output from M501:

20:13:35.057 : echo:Hardcoded Default Settings Loaded
20:13:35.057 : echo:Steps per unit:
20:13:35.057 : echo: M92 X100.00 Y100.00 Z100.00 E506.00
20:13:35.057 : echo:Maximum feedrates (mm/s):
20:13:35.072 : echo: M203 X500.00 Y500.00 Z500.00 E25.00
20:13:35.072 : echo:Maximum Acceleration (mm/s2):
20:13:35.072 : echo: M201 X1000 Y1000 Z1000 E4000
20:13:35.072 : echo:Acceleration: S=acceleration, T=retract acceleration
20:13:35.072 : echo: M204 S3000.00 T3000.00
20:13:35.088 : echo:Advanced variables: S=Min feedrate (mm/s), T=Min travel feedrate (mm/s), B=minimum segment time (ms), X=maximum XY jerk (mm/s), Z=maximum Z jerk (mm/s), E=maximum E jerk (mm/s)
20:13:35.088 : echo: M205 S0.00 T0.00 B20000 X20.00 Z20.00 E5.00
20:13:35.088 : echo:Home offset (mm):
20:13:35.088 : echo: M206 X0.00 Y0.00 Z0.00
20:13:35.088 : echo:Endstop adjustement (mm):
20:13:35.088 : echo: M666 X0.00 Y0.00 Z0.00
20:13:35.088 : echo:PID settings:
20:13:35.088 : echo: M301 P22.20 I1.08 D114.00

The endstops work correctly when I test them.

Any ideas on what is causing this behavior and how to fix it?

Thank you