Hard pressed to believe a bad coil would cause surging, if it was junk then you would surely have a CEL. I'd be curious if the ETM was playing up? If you can duplicate the issue and an indy shop close by I would get a second opinion.
Although not a Volvo, I just replaced a bad coolant temp sensor on a friends car, idle would rev up while in park for no reason, plugged the OBDII in and monitored real time data, witnessed the engine coolant reading going from -40F - 192F - replaced and all is well....worth a shot looking into.
steve
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