Ok the original tach for my dt3 was messed up. Some bone head kid laid the bike down. Summer of 83 if I recall. No matter, I bought one off ebay...was not for the exact model. Like it was for an at1 125 or something.  Fits fine. Wrong red line. It also bounces some. 
it doesn't seem to read right. Like I'll be putting around town and it's hardly working she running around 4000 to 4500 rpm....
I have a new cable.  What else could go wrong with a tach?

I have the original.  The top is off. It would need a face, glass, and the innards looked at.
The chrome itself is fine.
Also, the kid in this story was me hahaa

Any hints or tips,, lmk. If I decide to send it somewhere deet is already the guy. I just don't want to put a bunch into a tach that won't get more accurate if something else is goofy on the other end.

 

I've seen the plastic drive gear break (item # 4 in the tachometer gear parts list diagram). You might be able to see it if you remove the drive mechanism by removing item # 11. To replace the gear you don't need to split the cases. Remove the right side crankcase cover. This would be a good opportunity to do a clutch inspection/rebuild.

Worn or kinked tachometer cable will cause this as well as a defective tach.

You can also spin the tach up with a drill in reverse on the end of the cable and see how it performs when you vary the speed of the drill..  most likely the tach itself needs going through from what you've said?