Boxford VMC190 Retrofit Problems, axis sounds very rough

I have refitted a Boxford mill by removing the original control boards and replaced them with an optoport and spindle board from DIY CNC. Have kept the stepper driver board that uses three ST GS-D200s drivers the stepper motors, X and Y are 2.7A four wire bipolar Z is 3.5A eight wire in parallel (Nema 23 size made by sct 1998 vintage) and the power supply 24V 8A. It is controlled with Linuxcnc and everything works with one problem. All of the axis sounds very rough at about 50 to 55 mm a minute feed. The ball screws are 2,5mm pitch driven by 16 to 20 teeth pulleys and the motors 200 step per rev driven at halve step. The X axis was loosing steps so I tried to increase the current from the 2A setting (as set by Boxford) to the 2.5A max of the driver. This caused all axis to get worse and jump about in the same feed range. I dropped the current to X to 1.6A and it runs better without loosing steps but still sounds rough in this feed range. Have tried various step and direction timings and that makes no difference. Can anyone suggest if this is resonance, electrical interference or the age of motors or drivers?
The machine worked before I removed the original control using the manual controls on the front panel but this only gave ten set feed rated so may have jumped through this range.
Thanks for reading Mike

Have you tried another computer, as some computers can give this effect.

No I have not tried another computer but I don't think that is the problem. I have tested the output from the bob to the drivers with an oscilliscope ( and my very limited knowledge ) , the step line gives a clean pulse varying in frequency as I alter the feed and the direction one gives either 0v or 5v depending on direction. These signals don't change other than the frequency as I move through the rough running period.
Thanks Mike.

what make is the comp.?

Dell dimension 3000 with a PCI graphics card , pentium 4 2.8Ghz and 768 Mb of ram.
Thanks for your time Mike

dell's are usually ok.