I haven't used Windows in ages, but I'm pretty sure you can use the Task Manager somehow to see what's using your CPU/memory. I'd leave that up when you play, and then when the lag starts see what program is at the top of the Task Manager list.
If the program at the top is EQ, one of the other suggestions (eg. McCoy's idea of packet loss) might be correct. I think the Task Manager might show network traffic also, and if so you could check that theory in the Task Manager also, but I might just be thinking of the Linux equivalent.
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