When flying my craft and engaging the altitude hold module of the autopilot, mechjeb takes to wildly pitching up and down to the point of a stall. It only seems to happen on around 20% of my aircraft, but none of those 20% share any major similiarities that they don't also share with my entire aircraft library.

No idea what could be causing this. I've tried every fix I could find, and none worked. Any help would be appreciated, and I'd be happy to hand over any files you need. Thanks!

You might need to fiddle with the PID controls, depending on the characteristics of your plane. Speed control with slow-reacting engines might increase the problems, too. This test plane exhibited large oscillations until I tuned down the pitch i-parameter.

Ah, thanks! In my case it was lowering the Kp that fixed it. Thanks again.