First off, where is your tach pulling signal from right now? If you aren't pulling your tach feed off of the MSD box, then the MSD is very likely screwing up your readings if you're still pulling off of the coil. The tach feed under the hood is a blue wire that goes into the main chassis harness [if you didn't already know that].

If that's all gravy, and the wire is fed by the MSD box, then the ECU is already getting a direct feed off of the MSD, which leads to some more questions:

Do you have an aftermarket stereo equipment [specifically, amps]? If so, you may want to look into routing your power cables a little differently, or replace the tach feed wire with an EM shielded wire.

Also how long ago did this start happening?