Hi Martin....welcome to the Forum
My first thoughts are....which terminal did your mechanic remove first, only if it was the positive terminal then this can create a spike through the electronics. You should only remove the negative terminal first to isolate the electrical supply and not introduce any power surges through the system.
Additionally, if the electrics were troublesome before disconnection of the battery, then this could likely be an earthing problem which would account for the spike also.
Check the battery connections and earth leads first to see if there is any corrosion or poor connections before fitting any new electronic units.
Let us know what you find
Cheers, Trevor