1.6 tachometer jumping around after 3k rpm, or more exactly getting hung up.
Bad connection, bad tach, bad alternator?
I would go with bad connections at the alternator and maybe in the back of the circuit board. If You pull the cluster out you can put a direct connection to the signal of the tach and determine if there is a problem with the circuit board that way.
Hard to determine if the Alt signal is wonky without an oscilloscope I think. No idea how to check that unless it is just a continuity check with a meter.
Corroded male connection on alternator.
Thanks
Don't you just LOVE those simple fixes?