Also, I've noticed in more than a few of the small GA planes Ive rented that the altitude encoder on the ancient Mode C is not up to the task and is off by several hundred feet and floating a bit.
Its happened enough that when I get an unsolicited barometer reading (ie "hey buddy watch your altitude"), I'll ask what altitude he's seeing on his screen so we can compare and I can hopefully reassure him that it's the encoder and not me.
Fix