Two possibilities come to mind. First, on many Panteras with electronic ignition, the stock tach doesn't work AT ALL unless you use an isolater such as what MDS sells. Then, almost all work just fine. So I'd try to borrow a known-good tach & patch it into your car. No drive necessary- starting the engine should be enough. Note all Panteras and Mangusta tachs interchange.
Second possibility: the stock tach uses precision glass resistors (<1%) and I once found a cracked one inside on a tach's printed circuit board. Desoldering and replacement is simple compared to getting the bezel off undamaged so it can be reused, and locating a suitable replacement resistor.
Note the exact resistor value but as a non-precision component will change the calibration so you could be worse off than you are now. And fixing that is way outside my expertise. If a magnifying glass or VOM shows a cracked glass resistor or your tach shows signs of amateur work inside ( a resistor may already have been changed and picked now to cause trouble), try Mouser.Com. in the U.S. for a correct replacement.