Z-Axis problem - Not Enough Torque?

Hi, all.

I think I have a problem with my Z-axis not having enough torque and will
appreciate some suggestions and recommendations on what are the possible
solutions.

Following are the specs of my CNC retrofitted milling machine:

1) ZAY7032 Mill/Drill Machine, 2 HP spindle motor

2) X & Y-axis:
converted to ballscrews: 20 mm diameter x 20 mm lead pitch (4 start x 5 mm
lead pitch),
direct coupled to a stepper motor (Nema34, 4.2 N.m, 6.0A, 2.4Vdc),
stepper driver capable up to 7.8A, 80VDC, microstepping (2, 4, 8, 16, 32,
64, 128, 5, 10, 25, 50, 125) and presently set at 5.7A, 25 microstepping, &
37VDC
Mach3 - 250 steps/mm, 2000 mm/min

3) Z-axis:
converted to ballscrew: 15 mm diameter x 40 mm lead pitch (3 start x 13.3333
lead pitch),
belt driven (same ratio/no reduction) by a stepper motor (Nema34, 4.2 N.m,
6.0A, 2.4Vdc),
stepper driver (same as above) but set at 50 microstepping
Mach3 - 250 steps/mm, 1000 mm/min

4) Power supply:
Has multi-tap output capable of 37, 47, 52, 59, 65 VDC,
currently using 37 Vdc,
about 1 KVA

Presently, the X & Y-axis seem to be working very well with no backlash and
lost step. However, the Z-axis seems incapable of moving up correctly under
load or when cutting or milling. Also, noticeable chatter or vibration of
the spindle head when milling MS plate at only 0.2 mm downfeed and 100
mm/min feedrate.

I know my ballscrews are very coarse pitch but I have to live with it as
that is what is presently available to me. And hence, the reason I set up
the microsteppings to 25 (X & Y-axis) and 50 (Z-axis) to gain more
resolution. Considering that the X and Y-axis are capable of moving the
heavy X/Y table, is there any reason why Z-axis can not hold its position
under load?

So, are there any solutions or suggestions to solve the problem within the
present hardware parameters? If not, what hardware changes should I make?

TIA for all your kind response.

Best,
WT