Horrible Game Performance

I'm not sure what is happening, but I have the worst performance in the past week, which is very odd as I've had no performance issues since starting the game until now.

I have made no driver changes or hardware updates, no new software has been installed, I use disk clean-up, defrag, error checking, malware/spyware and Registry Cleaning weekly.  Other than MXO, I am having no performance issues with any other software installed on the system, games or otherwise.

I've checked my system temperatures during play and there has been perhaps a change of two degrees.  I have noted that MXO is using a considerable amount of my CPU, but I've never monitored its usage before (because I haven't had any real issues) and so I don't know what the standard there is.

Specifically, the issue is nearly non-existent while travelling outdoors or engaging in any outdoor activity.  It also does not seem to exist when going indoors until I take a mission.  Once I take a mission and head indoors, the game begins behaving erratically.  It will run smoothly, then suddenly my RSI will start moving like it's stuck in tar, then I will speed up again and slow down again.  It seems to get worse while turning corners and seems especially bad in Westview.... as a matter of fact, I didn't seem to have any (or hardly any) issues in Richland.

System specs:
AMD3000+ 2.7ghz
1GB DDR400 RAM
GeForce FX5600 w/256mb RAM
1 x 80gb HDD
1 x 40gb HDD
1 x 160gb HDD
DirectX 9.0c
Nvidia 81.89 Forceware Drivers

I've done a full patch and it seemed like there were fewer issues after I did the patch, but after about an hour they returned full force.

Any suggestions from fellow players or the devs/support are appreciated.

Hmmm... some good questions, Chuui.

Normally I don't die a lot, but lately I have been dying more often.  I've also noticed that there are times that I cannot use specific skills; I que them in interlock and they appear to go into the action section, but they disappear and are replaced by whatever default action is loaded.  I just noticed that last night.

Usually I do have the map on while I am doing missions, so that I can track areas that I haven't cleared.

I did notice snowing inside a few missions, but just at exit doors (ones that exited outside) and only occasionally.  Not lately, though.

One thing I have noticed that seems very strange is that the performance is worse when I set my cache to High; it is significantly better when the cache is set to Low, which seems strange to me.

It's especially annoying after having enjoyed such great performance and really enjoying the game.  Now, I can only play it for a limited amount of time before it just reaches the point where it's sucking the fun out of the game.