I have the same problems. I don't have the Apple bluetooth module in my G5, after calling Apple they said I could only buy a USB type one. I bought a $90 Motorolla EDR Bluetooth USB one, yet my mighty mouse still lags and turns off often.
I really hope BT mice are sorted out soon. Im aching to get one for my MBP, but i dont want a bloody base thingy, i just want it to connect to the laptop. But i havnt yet found a BT mice that meets the standard, especially with new laser technology.