Thanks for the replys, I would have replied sooner but i've only returned to work this week.
In response to Gareth we are runnining LEM on a Raid 5 array and do have quite a high number of events so it could well be latency causing the issues over time but it seems odd that sometimes once the time starts slipping it can repeatedly do so (talking in the space of a couple of hours) and other times be fine for days.
Also as you have suggested I upgraded today to 5.7.0 and applied the .jar file hotfixs to see if this helps resolve any issues. I'll report back findings after using 5.7.0 for a bit.