I have seen one other post that appears close to my situation and the resolution was a new immobiliser and/or Electronic Control Module.

The Car: 1995 960 with 110k and recently rebuild thermostat
The Problem:Occasionally and unpredictably the car will lose all power (once I stop the car, it won't start). In the worse case I get a power surge when driving (esp if going over speed bump or turning)and next time I stop it won't start; usually there is no indication when I am driving and I will stop the car and then can't lock the doors (using keychain) and can't start the car.
Symptoms: All power - radio, all interior lights, outside lights, and even the keychain door locks won't work. The ignition will turn over with no result. Typically, I wait a day or several hours and try again and it will start (radio goes to CODE). Often times I jump the battery to no effect (battery tested good, alternator tested good). I wiggle and jiggle wires under the hood and sometimes it seems to work. Point being it is not predictable as to when it will restart.

Volvo mechanic (whom I feel is solid) cannot recreate the problem because it always starts before I get it to them or even if I tow to them dead it starts when they try.

I love the car and this mystery is terriblebecause it is not repeatable or predictable.

Advice? Thanks

 

Check all your fuses under the hood