hot servo motors, heatsinks??

Greetings,

I am currently at the point where I am testing my CNC
conversion. I notice that the servos get quite hot
after about 5 min of running. Not so hot that I cant touch
them, but if you grab one, you can only hold on comfortably
for a few seconds.

I know they are running at low RPM, so this is probably one
of the causes (higher I^2R at low rpm?). Also, I am dumping
many amps into a relatively small mass, so heat is to be
expected.

My question for those with experience with CNC servo motors.
Does anyone install heatsinks on them?? At what point should
you shut down to let them cool off? I think they are ok
after 5 min, but if I were chugging away on something for 30 min
or so, I think they might burn up.

Some info on my setup...
This is a HF 47158 micromill with 190 oz/in servos
using direct drive (bracket and shaft couplers). I am
running a Camtronics/Gecko320 servo driver box, and also
installed a 20lb gas spring to offset the head weight on
the Z screw.