When flying along with AI aircraft, sometimes the AI will make S turns. This seem to be with any aircraft used as AI, flight plans made with AIFP. I have experienced this on longer, 500 to 1000nm IFR flights. Happens when cruising, the climb is ok. Happens with any weather settings. I have listened to ATC, and they are not being told to change course. Any ideas why the AI do this? Or is this normal FSX AI behavior?