It seems like the RAM is the issue then I'm afraid
If its faulty and still under guarantee, I don't think you should have any problems, but its good RAM what you have there, and so you shouldn't have to pay for it again.
Do they have some kind of longer guarantee with the company themselves? I have a feeling they do, so it might be best e-mailing them or phoning them to find out what you can do, or try going straight back to the company you bought it from.
Don't they say that if bought from the internet and it happens in less than 2/3 months that the fault was there when bought and so a refund is very easy to do? I don't think you've had the RAM that long have you?