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Have you checked the settings in the Logitech Control Center?

I Steermouse instead of the Logitech drivers as they were historically very poor. Is there a setting in there that tells the mouse to speed up randomly? ;-)

I'll install them to see if there's a difference however.

Cheers,
Adam
 
I Steermouse instead of the Logitech drivers as they were historically very poor. Is there a setting in there that tells the mouse to speed up randomly? ;-)
There is a tracking speed multiplier setting. I know some had problems with earlier versions of LCC, but that's very old news. I've never had a problem with it on multiple Macs running Leopard, Snow Leopard and Lion.
 
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