duffy,
One thing What may be the problem is that the output transistor (Q1?) spends a lot of time in the linear region, before the relay trips OFF. And it is still in the linear region even after that. I watched that on the o-scope. With just the relay, it's no big deal. But, when zone added the motor, as the relay tripped OFF and the motor stopped it probably allowed supply voltage to climb, increasing the voltage for the relay, and causing the relay to trip back ON. This restarted the motor, which caused the supply voltage fall, tripping the relay back OFF, stopping the motor, increasing the supply voltage, tripping ON the relay...and on and on and on. That was the chatter on release, Eventually C1 will charge enough to take the transistors into cutoff.
I think the solution would be a "stiffer" power supply, or a separate supply for the motor, or a switching circuit that is more absolute in transition...think Schmidt trigger.
Ken