Yes! I woke up this morning and began to believe it was network related too! Here's why: I can almost 100% guarantee that this problem will occur when I've left my computer on overnight with torrents running (this time I was using BitRocket, before it was Tomato Torrent). I have a feeling it is also upstream-related because I only had one torrent downloading. It was only around 100MB, and it finished just before I went to bed. So performance was still good when it finished and started to seed. I have an auto-stop when ratio hits 2:0, so after I seeded 200MB, there could be no more peers connecting. So this problem must happen fast, with only a few peers connecting over a short period of time. I should also say that I have Little Snitch running, and I use it in demo mode (so it stops after 3 hours into the night). I have no idea if this has anything to do with the problem at all, but it's just some more info to throw at you all. Also, one last thing I've noticed is ALWAYS associated with this problem is this console message that repeatedly occurs throughout the night:

The number of "ticks" differs, and it seems that the higher the number, the worse my mouse movements and GUI will perform. I've had it go up to 15 or 20 in the past. Also, the more frequently this message occurs, the worse performance is too. For instance, right now the mouse and GUI are slow but not as bad as usual, and I noticed that the tick numbers are all around 3-4 and the message only occurred a total of 5 times at random instances throughout the night/morning.

...now, for the cherry on top...I found this little line in there as well:

skgeosx is the network driver I am using.

I am using the same network setup as Snow. Marvell Yukon 88E8001. I too have the nLan adapter, but I haven't been using it because I had problems with it in the past. I am running 10.5.2 rev1 right now, so I don't know if it would work better now with this release. Maybe I'll give it another try. What network adapter/driver are you guys using?