No if using Metric then 320 is quite low but don't go higher yet untill problem is found.

Is it always the same Axis that goes wrong.?
When you say it's missing steps then stalling motors are you sure this is happening or just presuming because motors stall.? Can you stop it after a while of cutting and see any positinal error.?

Has it just started doing this since you re-wired or was it doing any of this before.?