[caleb-charles]

Simple things to try
1. Move or Shield the power to the motor – make sure the motor wires are not less than 10cm from the probe connections. Check grounding etc. – Thry linking the probe body to the motor case. If its already joined try disconnecting it (These things are seldom rational until you’ve solved them)
2. Put some capacitors across the motor contacts – 0.1 uF low impedance type should reduce most spikes. Wrap the power leads round a toroid or use series inductors
3. Move the motor – Drive the mechanism via a belt and pulleys (Usually necessary anyway to keep the motor out of the water)
4. Make Sure the motor power supply is not contributing – Some switched mode power supplies can kick out a lot of noise.- Do you get spikes when the voltage is reduced and the motor not turning? If so its not the motor…
Good luck