My timing mark is off but the motor runs better?

I have a 1978 buick regal with a 383 stroker. The motor was not running 100% so I went to time it and the timing mark is about 90 degrees advanced. The closer to the standard mark of 10 degrees advanced it gets, the worse it runs to the point that it will not run. I got the motor running good at this weird timin but I would prefer if the timing mark was right so I could perfect the timing. I had the motor rebuilt last summer and the machinist marked the harmonic balancer and the timing mark. The marks he had placed worked perfect for about a year, and than this happened. Any help would be GREATLY appreciated.