need motor for CNC (sherline)

I have a sherline CNC set up from Backtrack. The Z axis motor is
shot
(works with no load but just sits there "buzzing" when hocked up to
the mill). I have pulled the motor off the lathe and it works in its
place--so I think its a good bet that there is something wrong with
the motor. This is the second motor I have had and Backtrack charged
me over $160 for it.

Now I see a LOT of fokes talking about a LOT of different motors--
and
I don't understand much of it. So while I'm sure every thing I ever
needed to kow is barried in other posts, can someone suggest what I
need and where to get it?

The motors I have have a mount that has holes spaced about 1 13/16
apart and a 1/4" shaft on both ends. the markings on the motor are-
Mycom ps468-21B 5,4V 1,5A #9845 1.8 deg step and it has a 5
pin
DIN male conector.

I would like to possibly get something a little stronger as I have
constanly screwed up parts when deep into the program the the G00
command dosen't lift the head (just sits and buzzes) and then the
move
command ends up feeding the tool into the part for a cut I don't want.

Thanks!

Craig