68V is perfect voltage for 4Nm 23 so don't change. The MSD980 where not bad drive and can handle 80V so no issues there provided setup and wired correctly.
So provided setup correctly regards current etc then this shouldn't be your issue and you'll have plenty of power. So can rule out lack of power.

If your sure that you have no mechanical issues and Stiffness isn't so bad and all electricly sound then all thats left is the dreaded parallel port/PC. (I pressume your using PP.?)

First thing to do is change the Parallel cable. Not uncommon for parallel cable to give problems.
Next is control software, if Mach3 then I'd back up your XML file and do fresh install. Again I've seen mach3 do strange things and re-install cure them.
PC would be my next swap out. If possible I'd try another PC.

Best of all is to dump the PP and fit motion control card. Preferably Ethernet based. USB is better than PP but I'd avoid if can afford it.

If Still have problems and does turn out to be stiffness/weakness then I wouldn't try to make silk purse out of sows ear. Just sell it on and build another better machine.