Jumping Tach Issue
 

A friend of mine is doing some work on a '99 Miata but it has a crazy jumping
tach issue like the one found here: http://www.rivercityroad.com/garage/badtach.html

He says the engine misses really bad when the tach starts jumping around. He's checked the current grounds and has added extra grounding, but to no avail. It now has new plugs/wires. He says he is going to try another set of coil packs and maybe an alternator.
Any other ideas?