Engaging autopilot makes plane pitch up

[Pericles]

As soon as I engage the autopilot the plane pitches up huge (vertical speed flies to 8000 fpm), speed drops to stall, and no matter what I do I can't get the speed up once the pitch settles down. My display says "AP IN PROT" at first and then switches to "A.FLOOR." Any idea what those mean and what I might be doing wrong? I calibrated my throttles so that's all good. Thanks!

[Pericles]

Ok, I think it's related to my airspeed. I can't get the plane past 220 knots or so. Tried setting thrust at different notches, etc, nothing works. Even with engines running at 93%, plane won't move. So then when I hit AP I get the huge pitch up, speed reduction, and stall. Is there something simple I'm missing with the thrust/autothrottle?

[tomabram]

Exactly the same issue here, no idea what is causing it. 5 out of 5 flights its happened on.