Can someone help me with this?

Although I have quite a high end computer I continue to have low frame rates in certain circumstances, which I never had in a smaller PC in the same situation.I use FS2004 as my main game. Although FR is good most of the time, when I do spot view, after a couple of minutes, the computer freezes for a few seconds and then is back to life, but keeps doing the same until I change the view (happens only in Spot and VC views). In spot view, also the screen flickers from time to time (Render to texture is off), and I have black, lightning shaped, things crossing the screen from time to time. Also, when I have a lot of clouds (I use ASV6) the FR can drop to the mid 10s. This is totally odd, as I am running a FX55, and in my previous rig, which was a P4 3.0 I never had any drop of FR with clouds, no matter how many of them I had around. In both systems the VC was and is a PNY6800 GT.I use FS autostart (shuts down all unnecessary programs and services) and defreagments memory.My setup is as follows:FX55 processorGA-K8NSC-939PNY 6800 GT (AGP set at 8x)2 Gb Ram, Kingston 400Mhz, PC3200 installed as dual (the are the same manufacturer's piece number)200 GB WD HD, partitioned in 130 and 70 + a 80Gb WD.Audigy 4 Sound card (on board disabled)1 CD writer1 HP DVD writer.Compaq 7550 17' CTR monitor, set a 75Mhz.XP Pro with SP2 and all patches to date.I use AA 4x and AS 8x.I used Nvidia chipset drivers for the installation, except the IDE ones, that for some reason produced repeated crashes at start up.Ge-Force 81.98 (I tried the newer ones, but with the same result)For maintenance I run Registry Mechanic and Diskeeper (before launching FS2004).Anyone has an idea of what can I do to get rid of the freezing, FR, and blck lightning problems?Thanks,Leo