Sounds like you're not having much luck (an understatement i'm sure) - but at least OcUK appear to be trying their hardest to identify/rectify the problem.
As you state that you've been having problems for weeks, i'm guessing you've tried all the obvious tests - (is their thread link?) - and the conclusion you've come to is that you have an unstable clock rather than a rogue piece of hardware, for whatever reason?
If this is the case, have you tried reducing the clock to 3.5/3.8Ghz - just to check if you can get the rig stable with a moderate clock? (or even stock)
I realise you've paid for a 4GHz bundle but if you're now trying to solve this problem yourself it may be wise to work from a stable clock foundation and methodically work your way upwards to 4GHz. (You could post your finding/settings in the customer service forum and they will be able to advise you best on your next move/setting once you've achieved a stable environment.)
It maybe that they will want the rig back again – but I suspect they would be happy to try tweaking your setup from a solid foundation, if you’re up it, as they should be reasonably confident that the rig will eventually hit a solid 4GHz with the right nursing.