Speed regulation for universal motor
You can still use a SCR control, this would make your universal motor to run like DC powered, but only one half period conduction. The second semi-period is used to measure the BEMF from the motor and to set the phase angle.
Now probably you already have such control (post the schematics). If it doesn't work correctly it is possible due to incorrect setting (gain), or maybe yours is just too simple.
Another possibility is to add a tacho to the machine and then use a phase angle speed controller like TDA1085.