bug with the cursor

Hello ! Sorry for my english I'm French.
I have a bug with the cursor only when I play at Minecraft.
When I move the cursor (the computer mouse) the fps decrease until 14-15 and when I don't move the cursor the fps are at 50-60.It is not the computer mouse because I have already changed it. When I use the keyboard the fps are at 50-60. Once that is done one week that it lasts. I have no mods and not change of Minecraft version. I have all try but ineffective.

Config :

• dual core 2.0 ghz
• 2gb ram
• graphic card : HD 3850 ( 1/2 GB )

Cordially,

Thomassadi