Over the past couple of months I have been trying to do some recording. My set up is Guitar > UGM96 audio interface > Garageband

I found after 20- 30 mins I got a crackling noise when playing. After research I played around with the buffer size which did nothing and then I set up a separate account with the intention of only running Garageband. No other programs open up I even disconnect from the internet. This improved the situation a lot. Now I can play for up to an hour before the same crackling noise, less intense, occurs.

Any tips on reducing this to zero crackling??
It could be the amount of RAM that you have. I am not 100% sure of that but I thought I read that somewhere. So you might want to google that.
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How old is your computer and what is the speed of the cpu? That matters a lot more than RAM. Are you using a type of ASIO driver?

Also if your not connected to the internet you might want to turn off your anti-virus (or atleast make sure its not scanning). I am running a 3.4Ghz Quad Core CPU and 4GB RAM and if my anti-virus starts scanning I usually get crackling. With that said if you get a virus you can't blame it on me if you do this :p
Ha Ha! In fact I don't run an anti virus at all and have not needed too. I do the odd scan, but nothing comes up...

My specs are
Macbook Late 2008
Mac OSX 10.5.8
2.1 Ghz intel Core 2 Duo
2GB Ram
hmm well your specd don't look to bad at all. I guess really the only thing I can advise you to do is in the audio settings make sure you are using some type of ASIO driver and also check the min. requirements on Garageband. Sorry I can't help.