i have had this problem before, was due to 3 things either individual or in combination, first an exesive cutting load to the machine on one side this is obviously easy to spot. The second is as the motors get older the magnetism decays slightly, resulting in imbalance between them, this seems to cause a little resonance at slowst speeds, so the weaker one will faulter on steep accelerations, this is less evident with only one motor on axis. Thirdly also as electronic circuits and motors age, ie heat cool heat cool ceat cool etc. the sensitivity to control voltage may change slightly, this is more evident in analog electronics than digital but not impossible, and may result from power supply being very close to power needs (not enough headroom power), and particular repeatable situations is just enough to tip things over into error.
I would.. swap the leads from drivers to motors, if the problem is now the other side its your driver, if the problem stays that side its your motor or power supply. If all motors are fed from one power supply, try unplug the leeds to an unused axis(Z) driver that it using some ampres, and try, if now works better, the power sup may be a little small. I have come across more missing step problems manifest like these at ultra low speed with no load than when machines running hard.
hope this helps
ben.  |