CPU Maxing & Audio Crackling on Logic 9

Hi Guys,

I have searched this forum for answers but I'm not having much luck. Or they haven't solved my problems.

Specs:

Logic 9.16

Imac: 3.06 ghz intel core 2 duo, 4gb ram, version 10.7.3

Focusrite DSP 24

Up until about 6months ago everything was fine, I could run about 40+ tracks using audio & midi without any problems. Recently the CPU keeps maxing, audio/midi keeps crackling and this is only using 2-3 tracks of midi. I'm going round in circles trying to work out the answer. Nothing has changed on my mac, apart from a new HDD - I made sure the new drive was fast enough before I bought it, so I doubt this will be the problem.

I've tried:

Clearing the cache on the imac

Clearing Library Prefs

Fresh install of Logic

Adjusting the buffer

freezing tracks.

I am also not boot camped.

Any ideas????

Cheers!