Hi,
A couple of thoughts:
1) Is it possible that the aircraft is stalling because you've not got sufficient power? On some AP modes, the aircraft will following the glideslope by adjusting the pitch. If power is set too low, it will be descending at below the glideslope rate. In this case the AP will command more and more pitch up until it stalls. I've encountered this myself when tuning the autopilot for the FG1000 C182T.
2) Are you approaching the glideslope from above or below? Normally you approach it from below from level flight. If you are approaching it from above, it's likely that the AP will overshoot, descend below the glideslope and then have to make a huge correction.
I would suggest ensuring you are completely familiar with the 172/182 and how the autopilot and ILS system works before moving onto more complex aircraft. That way you'll ahve a much better understanding of what the AP is doing. Just like in real life
.
-Stuart