FSX Freezing Up During Flight

Lately I've been experiencing FSX freezing up during flight. By this I mean shortly after cruising at assigned altitude the game just freezes up and the Task Bar states that the FSX is not responding. Of course that ends the game. Oddly however I don't experience any problems when flying around a city and making touch and go flights and this includes any type of aircraft. The scenery look normal. I re-installed my FSX with Acceleration using previously published instructions and I also updated the driver on my video card. I didn't have this problem before and I already got the settings at normal and the Frame rate down to 20. My mobo has a PCIe 2.0 bus and I understand that I am restricted in using PCIe 2.0 GPUs, is this correct? What went wrong or what can I do about it except don't play FSX for extended flights. I can use some help.