Neil, soloing one track at a time may not expose the problem. Most audio clipping and distortion comes from gain. Thus your combined tracks may exhibit the problem while individual tracks do not. Here is how I mix.

In the mixer mode turn all volumes to zero. Then bring up the drums followed by the bass then the other tracks one by one. Carefully watch your output master while doing this. If it shows in the red start over.
Your song may not be as loud as you are use to but the clipping and distortion should be gone. Then you can mix master effect in, usually EQ & compression, again carefully watching your master levels. Note some use a pre-master channel for this.

I hope this helps and good luck.

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