Wow! I could have just as easily written this. Enjoyed FS9 with Just Flight's Air Hauler for about 4 years until personal issues completely drove me away from both. Now, I've built an AMD/ASUS 8 core computer because my previous build wasn't up to the task of Win 7. And I went with an 8-core CPU for future considerations.
Also, wanted to get into FSX and get back to Air Hauler. But for some reason FSX couldn't handle my 8 core CPU, or vice versa. While flying, FSX would slow right down to 1 FPS, but only after flying perfectly for about half an hour, and while landing at airport "X". Core usage indicated ALL 8 cores maxed out to 100%. ?!
Searching for solutions yielded nothing that wasn't over my head. One person said that they had the same problem until they bought FSX-SE. So, I sunk $20 and downloaded SE from Dovetail. Have been running it with AH for a couple of weeks, and NO bog downs.
For flight planning I've been using FSTramp. Unfortunately, there is no one-time cost. More like a yearly subscription. Sort of like the old FSNav for previous FS's.
Good luck with your own adventures.