We’ve just had 2 Garmin 275’s installed in our M20J 201, in the AI and HSI positions. Existing avionics at the time of the install were: KAP150, GTN650 and GTX345. Since the install, the 275’s seem to perform as expected, providing nav guidance, including glide slope and all other 275 functions, including the auto-slew. However, whenever the KAP150 is engaged, and put into heading mode, it will briefly respond to the HDG bug correctly (i.e HDG bug is moved to the left and a left turn is initiated), but it quickly assumes a much greater than standard rate turn (in excess of 30 degrees) and continues past the assigned HDG bug bearing without hesitation. It also begins a rapidly increasing descent in the turn, ignoring the altitude hold function, leading to a descending spiral. Turning the HDG bug back to counteract the turn, does not seem to correct the problem once it has been initiated. To say the least, this can be a startling development, that would be catastrophic in IMC. I was wondering if anybody has experienced anything like this after 275 install. Any suggestions would be welcome, as the plane has been essentially grounded for weeks and weeks as our installer tries to troubleshoot the problem.