Have your radio code handy. Loosen the ground on the battery, then without removing it, turn it back and forth a few times. Tighten it back up. Do the same with the positive cable.

Note if there is any corrosion at the battery posts and cables. If so, clean them off with a wire brush and enter the code on the radio.

There is also a remote possibility that the wiring at the starter is loosening up. A little harder to get to, but you will need to check the connections. Just make sure you disconnect the battery before fiddling with the starter connections - the wires are hot at all times.

Klaus
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Proud owner of a 220S. If I had more room, I would have more Volvos.