one of these keyboard/mouse combinations (although mine was the Bluetooth cordless version) and found it to be excellent - it was certainly the best-looking keyboard I have ever seen.
On the odd occasion I had the same problem, and I put it down to the batteries - in my version the mouse batteries were rechargeable and the keyboard ones weren't. I was so eager to use the kit (which I was testing) that I probably didn't give the batteries a full charge before I used the mouse.
The problem was sporadic, and hardly a real worry, but then again I knew I wasn't keeping the equipment for long. Had it been mine I might have been more concerned. I hope you resolve the problem from a selfish point of view - I was thinking of buying the Bluetooth combination for myself!