Few things
Type of driving really effects the SS system, age and type of battery has an effect, if its over 3 years old its possibly in a poor state, the sensor for the SS often fails(wiring) beside the neg terminal on the battery.
Very well known issue with the the wires going to the tailgate causing all sorts of issues, wiper not working, boot won't open number plate lights etc
Will the car restart if you intentionally stall the car?