I understand your lack of trust towards mechanics but without auto knowledge you'll need to find one that you can trust. Word of mouth is best if you don't want to trust a local dealership.

I'd still suspect a battery connection problem to begin with being that it's a recent problem. It's poosible that the terminals or connected wires there are not as clean, tight as they should be.

Other than that, there's several other possibities such as PNP switch, ignition switch, starter relay or related wiring. You can try moving the shifter between P and N if it re-occurs as that might allow it to start if it's a bad PNP switch. I'd also look at the small "S" wire at the starter solenoid (small single plug under cover) as it could just be a bad connection. Otherwise you'll need to have it checked by someone who's familiar to auto electronics, preferably a Volvo tech experienced with FWD Volvos.