I would highly recommend scanning for viruses and malware as for example malware can use all your system resources and cause system lag.
Use a virus and malware scanner and see if you get any results.
I have tried a system restore, and a full boot scan, before, do you think it could be a faulty ram stick? as my ram was bought second hand
As you are having 95% utilization on idle it is either going to be a hardware or software issue.
It would only be a hardware issue with the ram if some of the sectors where corrupt so you only had X amount of ram available to the system (Very unlikely).
Have you tried the ram in a different system or using a different different OS?
I have a 4770, so cpu usage is only ever round 4% and ram usage is around 18% when booted but goes up to 99% after several hours
As stated earlier, this seems to me like memory leak from a program or software.
Check what programs are running in the background as something is using all your resources (could be malware)