The problem is that for some reason the FM transmitter is programmed to reduce its transmit power when the N900 is charging. There is a script called "fm boost" which increases the transmitter power even above the default level, but it is a pain to use since you have to run it every time you want to boost the transmitter power. Or alternatively you can install a modified FM transmitter daemon which is modified to unlock the full range of transmitter frequencies, but also sets the transmit power to max by default and doesn't reduce it when charging.