chopper drive - resonance problems - need ideas

Could use some ideas here. I'm having some resonance problems with my
chopper drive. Maybe someone else has been down this road. First a
topology review...

Looking at 1/2 the circuit - a centertap and two windings L1 and L2, it's
kinda like a unipolar exited bipolar drive - the chopper is applied to the
center tap with a high side switch and when it's on the current ramps up in
only the one winding whose low side switch is on but when it's off (90% of
the time) the current freewheels back through the other winding so It
actually flows in both windings during that time. The two low side switches
share a current sense resistor as they are never both on at the same time.

The high side switch is turned on every 62us (16khz) and remains on until
current limit is reached. I'm running a 40volt supply and when it steps,
the current ramps up to 4 amps in ~600us and then begins chopping. The
steps are very 'forceful - when it's stepping slowly step - step - step -
step, the motor goes TwANG - TwANG - TwANG - TwANG.

There are certain speeds where the resonance of the armature wreaks havoc so
severely that it looses steps with no load on the motor. Particularly bad
at about 4000 spm.

The motor is not mounted so the case and the armature resonate - if I hold
the case down tightly to the table it improves.

If I apply slight drag by holding the pulley it stops resonating and runs
good.

If I half step the motor it's much better but can still have the problem.

I tried adding about 10us dead time during the ramp up each cycle and that
improved it some but it then took almost 2ms to reach I_limit.

I can program the current limit so maybe I could try making the step at 1/2
current then go to full current.

Am I going to have to 'tune' the resonance out with smoke and mirrors
software?
Maybe some kind of harmonic balancer attached to the motor shaft?

Clueless...

Joel