I had a similar problem when I put an adjustable vac adv in the HEI on my 292. The problem was mention in the docs with the adj vac adv, and there was a limiter included in the kit. It's a known problem and there are several limiters available here are links;

http://www.73-87.com/7387garage/drivetrain/hei.htm

http://www.msdignition.com/instructions/Products/84281.pdf

I installed the limiter and adjusted it, problem solved.

Larry

Ignorance can be fixed Stupidity is forever