I am having this problem on my very first flight. My machine is a fairly low spec, i7 4790 @ 3.6Ghz, with a NVidia GTX 750 Ti. I run Active Sky and UT Live as my main background programmes. I normally run P3Dv4.3 at a fixed 25 FPS but following your set up recommendations I ran at an unlimited frame rate. After managing to get to the cruise I noticed my FPS fluctuates every 10 seconds or so down to 4 FPS for a split second or so, and then recovers, just as if one of the programs is looping, or injecting new data... So guess what, I shut down Active Sky, and the problem disappeared. I think the issue is Active Sky ! This program has also apparently caused problems for the QW 787 with random CTD`s. There is a P3Dv4.3 patch on the AS website which I have installed.... As I type my airbus is flying stable and true..I will fly the same flight again with AS turned off and report back..