Toyota Matrix Tachometer has gone crazy

I drive a 2006 Toyota Matrix, automatic, 142m. My check engine light is on,
but I haven't had it check because it frequently comes on and off for the
evap sensor in the gas cap.

a few months ago all of a sudden the tachometer would move around very
irratically. it started the first weekend that it was really cold here, and has
since come and gone.
At the times when it's not working, when i start the car the tachometer will
usually hang at zero for a while and then begin to flail around usually
bouncing between zero and about 4k. Occasionally it will make it all the
way to the highest range of the gauge, 8k, and jump around there for a
while. Then without any trend that I've noticed all of a sudden it will function
correctly.
There are also times when it works fine the entire time from the time I start
the engine.

Another disturbing piece is that about a month ago my temperature gauge
joined in and will now remain at zero the entire time the tachometer is
flailing about and then when simultaneously they will begin working
normally.

Thanks for any insights!