Can't successfully download nor resume mac os x mountain lion I'm sorry if this a redundant question but I'm really desperate.

I recently bought a used Macbook (5,1) from ebay that came preloaded with snow leopard, my main goal of this purchase is to start developing for iOS.

it's been a while now and I'm ready to dig in, as I was trying to get the latest OS release from the App Store I've been getting this weird glitch or bug;

the bloody OS won't be downloaded! It shows as it's progressing, but suddenly it will go hay-wire and stop at 50 something mega byte. Sometimes it would show that I reached a giga byte or so but then after I leave it for a while (an hour or two) it would show an error "failed to download or something"

I admit that I live in the middle east, and we might not have the best ISP's in the world, but I successfully downloaded 4 or 5 giga bytes before with no problem whatsoever!

What shall I do? I'm really relying on you to end this for good.
It seemed that the ISP was causing it. I took to an authorized retailer and it did it for me.