After a few more days I cracked it. The Skyrim random performance doping issue. Basically I had power saving mod enabled on my PC which lowers performance to save power, it can even restrict your CPU speed! Which explains why the problem appeared to be a memory leak. Anyway if you are experience random performance drops in your games with no explanation... it might me worth checking your PC's power saving settings by searching (Power management) in the windows start button an select performance mode. If this solution doesn't work for you, don't give up, nothing makes sense in IT, an nothing seems to make sense in Skyrim, but keep looking through forums like these, the solution is out there.