Auththrottle loosing speed in turns

I've come across a strange problem with the autothrottle in a couple of my payware addons (PSS 757 and Maddog 2006).Problem is, when the aircraft is banking in turns, the autothrottle seems to be unable to read the IAS properly. The engines start increasing or decreasing, resulting in the speed deviating from the commanded speed.I made a 90 degree turn in the 757 today. It was in VNAV mode, commanded to maintain 292kts. For some reason, during the turn, the throttles reduced, causing the speed to drop to 250kts by the time the turn was complete.Is this a known problem with certain aircraft and FS9? Are there any ways to fix it?