Bell 412EP AP Out Instability

[wazz'n'zoom]

Bell 412EP AP Out Severe Vertical Bounce

Just had the closest shave ever whilst conducting AP out hovering in a Bell 412EP with AHRS. On taking control from a Student pilot, who was slightly over-controlling the cyclic, the a/c started to bounce so severely in the vertical plane that vision was blurred- approaching G induced grey out, the helmet boom mike fell down out of position and the a/c information clip file in the Map Stowage slot burst open. As it was impossible to fly let alone land, I forced the cyclic forward (witnesses say 40deg nose down) and the bounce became even worse. The bounce was similar to a 'Computer Acceleration Control' or CAC runaway experienced on Lynx/S61/Sea Kings but a lot worse. Oddly none of the 4 crew saw any Tq swing or heard the engines hunting. The saviour was yelling to the right seat to engage the APs. Due to the bounce he really struggled to get his fingers anywhere near the AP Panel. On engagement the bounce dampened out almost immediately, a normal hover resumed and the a/c landed and shutdown 80m from the hover point. The whole incident took about 15 seconds and was witnessed by quite a few people who mentioned that they thought we were about to crash. Funny old thing-so did I!
Apparently the tailboom was flexing in all directions and to be honest, I'm amazed that it stayed attached given the severity of the bounce!!
The a/c was mid CofG, weighed 8900lbs and the wind was very light from the rear. Mast Tq was about 55% with no yaw. The pilot’s notes warn of an AP out condition where Tq and lever will de-synchronise at High Tq in an erratic yawing turn; none of these were met. The engineers could find no fault and the brave MTP took it for a spin with no fault found. This is the third 'odd' AP Out incident on our AHRS airframes. There have been no TARSYNS AP Out related incidents!
Anyone else had anything like this? I'm not convinced that we have got to the bottom of or heard the last of these potentially serious incidents!