I use a Digi-key 497-6469-1-ND Transient Voltage Suppressor
on every outside lead connected to the circuit.
I believe they are much faster than old zener clamps.
Also try to add a 100 ohm resistor or better ahead of each TVS so it doesn't
have to pull the spike down in the entire system.
For the power supply their could also be a diode and a 100 ohm resistor in
series followed by a large capacitor and TVS in parallel to ground ahead of
the volt regulator.
The 100 ohm resistor can get hot depending on the current draw.
You may have to adjust to less than 100 ohm to maintain voltage at peak
current draw.

Norm