Do you hear multiple beep signal from the PC (case)? What's the rest of the setup? Do you have multiple RAMs installed? If you do, then try to use one RAM module at a time and check if the problem persists using only one piece of RAM.
I had some trouble finding the item you've mentioned. It turns out to be a Slimline 3115.uk, not 315.
Well, how long have you purchase the PC? With 160W of PSU capacity, I'm not really sure it's gonna hold long. My guess is the PSU is failing, since the symptoms you described are pretty much similar to that of a failing PSU.