I had this same issue a few years ago when I went to an electric water pump and switched to just having the belt run from the balancer to the alternator...it would throw the belt EVERY SINGLE PASS down the drag strip. I tried aligning pulleys on it for hours...didn't help. When I would race I would have to bring at least 3 extra belts, because sometimes I would be able to retrieve the belt when it flew off, other times it would be gone or destroyed by getting stuck between the balancer and the timing chain cover.

Long story short, I solved the problem by going to a wider and stiffer belt. The belt does not fully sit down in the groove now, but it does not twist and fly off either. If I recall correctly, I had to go with either a 1.0cm or a 1.2cm wide belt to make it work (I started with a 0.8cm wide belt). Good luck.