Mice and Pointing Devices

[Julylol]

Hello, I was wondering if any one could help me with this, over the last week, I've been having problems with my logitech mx518, whenever I'm in game, and even sometimes out of game, the DPI just changes and the "gliding" feels heavy, I try to change it back to what it was but it just changes back after a while, I've been scratching my head and don't know what to do any more, here is what I tried so far:

1) Reformatted my Computer

2) Installed drivers + messed around with settings + reset back to default

3) I actually bought a new MX518, still same problem so must be driver/software related...

4) Got a new mouse pad incase the old one was worn out

5) Switched USB driver ports.

6) Unistalled Drivers, using standard Windows 7 Drivers at the moment.

So does any one ever experience this ? It's really hard to play games at the moment because my DPI just changes and it messes up, I play very fast paced RTS games where you move the camera with the mouse mainly and you use the mouse to click on little units and "things" and this can mess up gameplay a lot.

Any one got any suggestions ? I don't know what to do, it's been a week trying to figure out what's wrong with it.