Several ways I can think of to cut down on interference.
1. Can the decoder and motor supplies be separated? If not use the decoder to switch a relay that then uses a different supply to operate the motors. I've done this for other reasons, using latching relays, they then remember the last position and can be used by pulse output decoders.
2. Fit VDRs (voltage dependant resistors) across the motor connections, these will cut off any high voltage spikes, I had to use them on some Fulgurex motors which messed up a hardwired signal system.
3. If voltage is dropping when motors operate fit a larger capacity supply if the voltage drops at the power supply. If the power supply end is ok run larger cables to the decoder / motors. We had this on a modular layout where the wire runs were long or used small wire.
Mike
