[Blas]

Don,
I'm not well versed in fuel injection control systems...
Lets check a couple easy things first.
I'm assuming there is no drop-off in power when the tach goes weird...
Check to be sure you have a good, clean, tight ground and power wires to the MSD box...(These are the red & black heavy wires to the box)
Check all the wire connection crimps in and out of the MSD box too. FYI: It's usually a white wire feeding the tach from the side of the MSD box.
Can you check the connection on the back of the tach to be sure it's seated properly? Carefully thread your hand up there and lightly push.
Pop open your distributor and look at the center carbon electrode...does it look like all the rest? Not worn down or burned looking? That would be because the connection to the rotor is arching if it was not touching the carbon button, or because it's bent up too much rubbing the button off.
Let me know...we'll go from there....