Mine was giving similar symptoms. In my case the earth strap had failed. Only found out when it let me down.
It attaches on a lug near the starter motor down the front of the engine and runs to the near side inner wing. Mine failed inside the strain relief moulding at the starter end so wasn’t obvious.
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2009 Volvo V70 D5 (Polestar 225bhp) r-design 180,000 miles
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