Upgraded to HEI and now the tach jumps around

I upgraded my ignition to HEI over the weekend, and now my tach jumps through a range of 800-1000 RPM. Tach is an old Sunpro, connected to the negative post on the new coil as it was with the electronic ignition I replaced. Tach was rock steady with old ignition. What other information does anyone need to help diagnose and/or what should I look for?