A misfire only at low speed is unusual. Ignition problems usually show up more under heavy load or high RPM. Does it idle smoothly with no load on it (i.e. could it be a vacuum leak)?

I vote for the fuel system cleaner. Injectors could be spraying odd patterns.

The pinging sound is the sound of metal parts slipping on each other as they cool at different rates. Usually, it is the sound of heat shields cooling faster than the underlying catalytic converter. Volvos don't usually do this, which makes me think that perhaps a lot of fuel is getting out of the engine unburned, so it is burning in the cat, causing it to get very hot, which results in the noise you hear as it cools down.