G320 Keeps Losing Position!!!!

Hi all ,
Recently I've installed two G320 drivers on identical x and y axis DC
motor ,
the Y work perfectly but the x axis motor loses position I suspect
noise problem : the only observation I can tell is that the motor
sounds like hurt pulsing when it stops , If I move it a little the
pulsing sound stops for several seconds then restarts !!!! Is That due
noise ?
I grounded the shield at the driver side , and shorted the cable to
minimum (about 5 feet) but no effect , I replaced the encoder also no
effect !!! , I switched the drivers it keeps with the x axis motor !!!

What other factors I'm missing here Is it noise or maybe a faulty
motor?!!!
Can I put a pull up resistors for the AB encoder channels? does this
supports the signal ?!!!

I'm totally lost and this machine has to be working soon So pleas help

Thanks,
Shadi