[jono not bono]

Check the buffer size... The higher the buffer size the more latency, but will (providing your computer is fast enough, and equipped with enough RAM etc) usually sort this kind of crackling noise. It could also be your hard drive being to slow...

When you look at the transport panel, on the far left there are two blue thin meters. The left bar is processor power being used, and the one next to it, on the right, is hard drive.

Are any of these two meters going up to the top and hitting red?

I can't really suggest much more without knowing more about the problem.

Are the 5 tracks in use midi channels using VSTi's? Or are they channels of audio or samples with many plugins or no plugins?

Sorry I'm not more useful

Jono_not_Bono