NeX:
the AC voltage then goes through a diode bridge and into a big ass capacitor and then on to a 220v DC motor. all of this circuit was pre existing for the motor.
That may be your problem. That capacitor tries to level the voltage, so every time you open the TRIAC it tries to charge, causing a current surge. The longer the cuts in the supply current (i.e. lower duty cycle) the more this may become an issue.
Isn't it possible to install a MOSFET in the DC side (which, by the way, should be more like 320-340V: normal AC line is 230-240V nowadays, times 1.42 for the smoothing), so you can use that to regulate the current? I don't know if it makes sense to keep in sync with the AC power supply, though.