I too have had the same issue with Dyno Jets and I think it has to do with the wasted spark. They use an inductive pickup and you start to get cross talk from the "dead" cylinder ay higher rpm. It's a radio frequency interference issue, not a ground issue. Using the negative lead on the ignitor as the LM2 does avoids that issue.
2008 Thrux. 1087 TPUSA motor. For Sale in the classifieds