Misfiring! Oh, have you read my V70 P23 misfire threads? It turns out I had dodgy coil packs, changed them and now it's fine. Get a code reader off ebay (OBD2) and read your codes. My bet is P0300 or P0301, P0302, P0303, P0304 or P0305 for a 5 cylinder engine. If it is, change your coil packs, they're a ruddy nightmare!