Sounds like a problem with the drive.
I'd personally just replace it with a servo.


A 110 frame servo is 110mm square, but generally have fixed connectors you'd need to allow for.
Smaller frame sizes often come with connectors on fly leads, so can be squeezed into tighter spaces.

Have a look at Lichuan servos - http://servo.xlichuan.com/ProductsSt_jlcfjl.html
They're available via their AliExpress store - https://www.aliexpress.com/store/1907567 and Jazz also stocks some Lichuan stuff.

Depending on what spindle control method you use (I'm assuming you're using 0-10V for the Sprint drive), you'll have to check what drives would work, as IIRC not all accept and analogue input. If you can use step/dir to drive the spindle, then I think any of the drives will work.