Three Parts To The Circuit...
There's the TDC sensor in the motor, the TDC sender on the fender well, and the tach itself. Along with the wiring to connect it together, the problem could be anywhere.
If you've replaced the sender with a new one, I would try giving it some time to adjust. Sometimes these things seem to fix themselves...
My next suggestion would be the tach itself, and then the sender in the motor. Good luck, and let us know what works!
"We drive into the future using only our rearview mirror."
- Marshall McLuhan -
Northern California Wine Country...
"Turbos whistle, grapes wine..."