Andz

Check your static system. Had a similar problem recently with the a/c porpoising 200ft with number 1 system coupled to the autopilot. A small leak from the hose to number one ADC was found.

Will do, although I know the ADS was replaced and a leak check performed, I will double check for leaks. The problem here is intermittent, I'm leaning towards wiring faults, but will look at any possible culprit!

Not that I know of. I'll ask Comm/Nav. Does it affect airflow?

GACFaja, just wondering, is it fixed? I have one E model that's got me bald after pulling all my hair...(not really, I shave what little hair grows) This one is intermittent and as far as I know it only porpoises on alt hld. I flew in it a few times this last two days and saw it abruptly porpoising on alt hld. Basic A/P was ok, as soon as alt/hld was engaged it would start a rodeo party. Could not see the elev trim trying to correct it, probably because the porpoising is so abrupt it does not calculate a correction.