And I assume the motors are kicked on by a relay...
Have you added a 'back EMF' diode across the relay's coils yet? The ULN2004 may have a back EMF diode built into them ( and I don't think they do), but I'm assuming that the ULN2004 isn't sitting right next to the relays either. Ya gotta have that diode in there. It's cause you pain. Have you thought about getting rid of the relays and switching over to a solid state relay?
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