[richiebee]

Are you using a compressor on your master fader?

It could be that your computer can't keep up with all the processing it needs to do when it mixes down. Try bouncing in real time. If that doesn't work, try exporting stems, bring them into a new project and mix the stems. It's not something you want to have to do on a permanent basis, but it will tell you if its your computer that's the problem.