What sends the signal to the Tach?

My tach jumps around when cruising 4500-5000 RPM.

I can hold it steady at 4500, and then go to WOT on the freeway, and it will start to go up, and then suddenly drop (instantly) like 400-1000RPM.

And the motor is fine (cant feel it lag or any power loss when it drops).

So i figure maybe a bad connection, or the sender is messed up?

Any ideas?