wireless mouse keyboard prob

I just got the cordless freedom optical mouse and keyboard set from logitech and it is behaveing oddly. When I'm playing games (counter-strike, quake, unreal tournament)there seems to be a noticable amount of time between when I click the mouse and the gun fires. Also when I try to hit more than 1 button at a time the thing won't do the second command. like in counterstrike, if I'm walking(holding shift) and go to crouch(hit ctrl) I won't crouch until I release shift and then hit crtl. It only happens some times but it really makes me mad when it does happen. I have fresh batteries in both the mouse and keybord and the reciever is only about 1ft away from both. I got them very cheap ($57) and they seem to be used or refubished. The s/n on both the mouse and keyboard are covered in black pen so I assumed they are refubs.
should I send them back or is there something I could do?
any help would be welcome!
thanks

comp stats if applicable:
pIII 800
512mb PC133
20gb 5400rpm
nVidia Riva TNT2 m64 pro
mini atx pos mobo
mouse/keyboard plugged in via PS/2 ports