Bad mouse lag while playing p99 (sometimes)

[Portasaurus]

Occasionally, I'll experience very bad mouse lag while playing EQ to the point that precision of mouse clicks is pretty reliably unreliable. When I am experiencing this symptom, the mouse jumps around the screen ~20 pixels at a time.

Why would this happen with 15 year old tech on a modern system?

The lag is pretty much exactly the kind of thing you see from either:

1) an overtaxed cpu
2) low wireless mouse battery
3) using an incompatible surface for the optical mouse

None of these three scenarios exist when the mouse does this. There seems to be no rhyme or reason as to when it starts doing this. I've had hours-long sessions without a single issue. I've had other days where it's caused me to borderline ragequit while trying to sell groceries.

The lag is reminiscent of using go-to-meeting or other screen-sharing programs across a slowish connection.

Does anybody else experience this behavior from their mouse *only while playing eq?*? Has anybody overcome this problem?

Thanks!