When I have full 3G service and try and place a call I cannot hear anything on the headset or it's very low, I have to turn on the speaker phone if I want to be able to have a conversation with someone, does anyone else have this problem, or know of a fix?

Did you try re-flashing?

Have you already tried using the hardware volume / zoom buttons to max the volume (while starting a call)? By default the in-call volume of my N900 was quite soft too, but now I can even talk on the phone while im in a bar / club etc.

I always have it at the highest volume, it will go high, then start to lower the volume, then back at normal again, to nothing.

I haven't reflashed it either, I have no idea what it does, or how to do that.