Just a thought sMoKiE, after replacing the batteries (with Alkaline or Li I hope), did you reset both your mouse and transmitter/receiver?
Next, try another new set of batteries. (I've experienced dead batteries right out of the package (not many in recent years tho).
I've experienced the same with batteries just before it totally stops. I'm trying Li batteries right now to find out how much longer they last and then I'll figure out if they are more cost effective.
Harder