If it works well with text editors and other light stuff, I'd say it's a heat problem. According to a short look at weather, at least some of Australia has 34 degrees Celsius right now... we very rarely have that in Finland. I've actually often wondered how Africans manage their computers!
There are two ways in cooling: getting cold in, or getting heat out, it really requires both. If all you have is hot, it's quite difficult to solve. You'd need liquid nitrogen or something, like overclockers do. I've thought about building a computer inside a refrigerator, a minibar... One engineer told that to get rid of noise problems as well as heat problems, he would submerge the computer in concrete!
Not an expert here, but somehow you should get chill in there. Another thing is, if you have a low end computer running at its peak, it'll create all the heat it can.
Warning: Going far beyond, water cooling or ground cooling? We use ground temperature to warm houses, perhaps Aussies could use it to cool?
You may not shoot me.