digital meter

I tried this thread on the Corvette forum but no one wanted to answer. If you read further on you will see it works well on my 70 Swinger 340. Maybe the Mopar guys are smarter or less stuck up than the Vette boys.

My actron digital meter will not read RPM on my 75 Vette with HEI distributor, it flashes wildly and will not settle down. I have tried the tach wire both in front of and after the capacitor and the coil ground. In all instances my meter negative was tried on the carb, engine ground and alternator ground. It works great on my 70 Dodge A body with 340 engine, electronic distributor and external coil using coil minus and carb ground. Are these new digitals not capable of functioning on the HEI units? My original dash tach works but I suspect it of being 200 to 400rpm high and I just want to verify that discrepancy and have an accurate tuning tool.
Thanks for any help, Jerry