Are we saying we use a tach wire at the spark plug wires or coil? If the source for the tach is the coil, I'd look for two wire eyelet ends at the coil. I'm guessing the two wire studs out of the coil shows three wires total used at the coil? One coil stud is keyfob hot to coil, the other two wires are piggy'd over each other and they are the tach, and pickup coil off of that coil stud. If those are rusty, corroded, maybe file fresh metal at the eyelets, steel brush the coil stud?
Since you covered the connections, changed out a good known working tach, there are only two things I can think of are; electrolysis at an ageing bike's wire ends, or a dirty piece of magnetic debris at the crank pickup sensor. This is assuming the tach is the remote processor receiving the crank signal. If it goes thru the EHC as aspen mentioned for the speedo, back to the black box being checked out.