Timing tech.

Timing Fluctuations
If your timing seems to be jumping all over the place, check the following items:

What kind of timing light are you using? Many lights have trouble reading the MSD's capacitive discharge multiple sparks. Dial back timing lights are not recommended. Try another non-adjustable light and check the timing again. MSD offers a Timing Light, PN 8990.

http://www.msdperformance.com/page.aspx?id=3306