Set up help please

[dmgdesigns]

I have search and can not find what I am looking for or I am not looking in the right place. Oh yea I completed the mechanical build of my 36X53 machine.
Mach2
Gecko 320
Us-Digital E3 250 CPR
Bob Campbell breakout board
Reliant E-712 Servos
PS 60VDC 30A
Screw
X .75 OD .5 pitch
Y .5 OD .5 pitch
Z .5 OD .25 pitch

Problems:
When I power up the motors jump and free wheel at a constant pulse in one direction. If I add a load like my hand or the screw the motors still jump and free wheel but not as far and some times jump the opposite direction. If I put a lot of load on the motors they will jump back and forth. I have tried tuning the Gain and Damping per Gecko's instructions - this is how I was able to get them to bounce back and forth.
I think I may be setting up the motors wrong in Mach2 The pulleys are 1.75:1 with a .5 pitch screw and a 250 CPR encoder. I have X & Y set up at 7000 steps per unit. I am using inches.
Thanks for any and all help!

[ger21]

It sounds like you have either something miswired, or you have noise on the encoder lines. Are they running next to the power lines?

Also, I think your steps/unit should be 3500. 2 tpi x 1.75 x (250x4) = 3500

[dmgdesigns]

The encoder lines are shielded and the only time they run next to the power lines is ine the cable track.

3500 steps per unit thanks

[ger21]

They shouldn't be in the same cable track, if possible. Not sure if that's your problem, though. Are the shields grounded at one end only?