All this on a dual core Athlon 4600 with a GeForce 7950GX2 dual GPU graphics card. And yes, I always set the affinity on my dual core Athlon.
Checking the debug output from the client, and I'm getting loads of messages like this whenever it happens:
INFO: Playing back delayed packet 36
INFO: Playing back delayed packet 37
INFO: Playing back delayed packet 38
INFO: Playing back delayed packet 39
or:
INFO: packet_out_of_order - got packet 1123 expecting 1106 from xxx.xxx.xxx.xxx:13
004
INFO: packet_out_of_order - got packet 1252 expecting 1232 from xxx.xxx.xxx.xxx:13
004
INFO: packet_out_of_order - got packet 1310 expecting 1286 from xxx.xxx.xxx.xxx:13
004
So I'm wondering if it's a network problem. I have reset my router multiple times, yet the strange behaviour prevails.
Needless to say, this is making it difficult for me to do anything, even writing code!

On a brighter note, sim crossings, when I eventually stutter my way across a sim, are now the quickest I've seen! It's almost as if one problem has been swapped for another! Oh well.