Its possible that your crankshaft pulley has worn so badly that its bouncing all over the place. There's rubber dampening material between the inner and outer edge of the pulley. If this gets soft it becomes impossible to set your timing using the pulley for reference. My 89 b230 has this kind of a pulley, and I had that problem. Check around for prices as they can vary significantly. I've seen them on ebay for as little as forty dollars. I paid over a hundred a little more than a year ago. An alternative is to buy a the solid steel pulley used on older engines. The dampening effect of the rubber core helps calm engine vibration, but it is a wear item.