Why does my mouse lock up?

Why does my mouse lock up?

This is a technical question about computers, hopefully someone will have the answer.

Whenever I have the Runescape game player client up (in the background, such as minimized) on my computer, my mouse cursor freezes, jerks and becomes laggy. So for instance, if I try to access Youtube or any other site, the mouse will make jerking motions across the page as if it's being controlled by something else. When I take the game player down, this stops. If I'm actually playing Runescape on the game player, there is no mouse freezes or lags on the cursor. It's as if the game player is controlling my mouse so that I can do nothing but be in game or have to take it down. My mouse is fine, it's not broken. Is there a setting on my computer that would prevent this application/software from taking control of my mouse?