It may well be the regulator, but as tumble says, it'd be a shame to buy / fit a new one and find the problem still persists. I'd be tempted to take a couple of temporary wires straight from dynamo to reg, earth and battery. A clip-on ammeter (or spae ammeter) or multimeter on battery would save the hassle of going through the existing ammeter and would prove the point fairly quickly. Also, make sure dynamo is actually earthed to the frame, not insulated by paint.