i had this problem once before myself (i have the servo dynamics amps). the problem ended up being the bias supply voltage to the drive was marginal. if you measure the voltage between pins 1 and 3 on connector J4 at your x-axis servo card and read something close to 28V you will likely be experiencing problems. its supposed to read 30V.
in my case, the voltage read 28.4 and the result was the x-axis behaved like you describe. the manual claims that less than 28V will cause the drive to simply shut down but 28.4 seemed to be marginal enough to cause problematic behavior. replacing the bias supply on the drive fixed the problem. the next time around i will likely be replacing the amps with something a bit more modern.