First, check the mechanics of your machine, usually that is the main problem, Mach3 is the last place where loosing/gaining steps can occur.

Second, do not run your spindle without cooling. If you are afraid of freezing, just use antifreeze. You can leave it in the cooling system even in the summer. Also, I would advise to check if the antifreeze contains any chemicals that prevents algae/bacteria. If not, you can add a little of potassium permanganate in solution.

A friend of mine just rebuilt a 10 yr old Chinese laser and added a bit of permanganate in the cooling water.