What could be causing my y axis to slip? Occasionally, while printing, my y axis will slip and the layer will, from that point forward, be shifted, ruining the print.

What might be the causes of an axis slipping? I have tried cooling the motor which seemed to have been getting warm, and the belts are not too tight.

This does not happen with every print, and seems to be an intermittent problem. 

My printer is a MendelMax RepRap, and the y axis is my moving bed.
(source: all3dp.com)
Your printer is skipping steps in the y-direction. This can have several causes. Take a look into Shifted layer guide on RapRap.org which lists 29 possible problems that can cause this issue and how to fix them.
First items of the list:

Driver current is too low
Driver current is too high
Belt too Loose
Belt too Tight
Loose Set Screw/Grub Screw
Belt or Bearing is binding
Speeds are too high
Acceleration is too high
...

When I was dealing with this issue on my RepRap I had to increase current to the particular driver.