This is a weird bug I've noticed in both my old system and new system.
I'll be flying OK with normal frame rates. As soon as I touch down, my Frame rates go to 1.6 FPS. They stay that way until I takeoff again at which time they are normal. The only way to fix it is to exit FS2004 and start again.
My specs are: CPU:AMD 2800+, MB: Asus A7N8X-E Deluxe, 1 GB Corsair RAM, ATI 9800 Pro (128)with 4.1 drivers.
I had the same problem with my old system which had a GeForce 2. Any help would be greatly appreciated.