While I have little knowledge of the Q400, I can say that I have experienced something similar to what is being suggested. ILS to 28 into ORD a few months ago, an SAS plane missed the ILS hold line and got into the path of the antenna. We were all systems normal at 4000 feet with the autopilot on and out of nowhere, the airplane took a swan dive after the glideslope fell out of the bottom. Clicked off the autopilot immediatley, but it happened rather abruptly and commaned a 2000 fpm descent as I was correcting myself. It does raise potential questions, but as I have said many a time on this topic, I will wait for the final report.