Weird FPS -Question-

Good Day Simmers: I have searched over the forum and did not find anything about my problem. I wonder if anybody could help me, because I dont find the way, I am setting my FSX and with all my sliders to the left (traffic, weather, scenery etc.) my FPS goes from 15 to 50 FPS, in KLAX and KSEA, on ground shows 15 to 20 FPS,after take off about 1000'ft, the FPS move to 40, the same in MMSD; 20 FPS on Ground and 50 FPS on air.My question is why?, is a big difference between ground and air with no visible scenery or traffic, and I have an Nvidia GeForce 8800 GTX, is supposed to have more than 50 FPS without Scenery, and all the sliders to the left.HP Compaq DC7700 (upgraded)Windows Vista Ultimate 32 bitsIntel Pentium 2 Core Duo @ 1.86 Ghz (E6400)2 Gigs RAM @ 667 MHZ2 Sata 160 GB Hard DrivesPCIe Geforce 8800 GTXSamsung 24