Floaty mouse when unstable fps

Recently I bought a new laptop (acer AS5560) with windows 7 installed and theres a very annoying problem I have with First person shooter games.

Basically, when my fps ingame is unstable my mouse is very floaty and unresponsive, its unplayable, really. Its like vsync but worse. Funny thing is, if I cap the fps so that it remains stable at all times, the mouse is fine, responsive and there's no input lag what so ever.

the vga on the laptop is the ATI 6520g and the mouse im using is the logitech mx518.

the mouse is fine playing non fps games, LOL for example.

that sounds pretty common for LAG specially with a low end cpu, the game is taking up all the cpu power when its lagging like that therefor your mouse input is also freezing/lagging

Cap the FPS. It's not a $2000 laptop, not much you can do.

FPS takes a lot of good stuff for great playability, sadly that laptop doesn't have a ton.