How then had it run 7 hours CPU time after at least 14 hours with no other project taking resource and only crunched about 75%
Well....the answer lay in an obscure interaction BoincManager 6.3.21. This box is Ubuntu64, so it may not apply to Windows machines, but BoincManager was draining a core leaving only half a core to run two WU's.
Looking at the system performance confirmed this and running top in a terminal window also confirms it.
Closing BoincManager allows Boinc to continue to run, but stops the drain on CPU resource.
I then went to look at all my other Linux crunchers running 6.3.21 and was horrified to find the same drain on CPU
I hadn't noticed this previously because the earlier versions of Boinc for GPU were known to have scheduler issues and my CPU project crunch times did not easily compare across hardware and boinc versions. It was only when I started a new project on a box from fresh that I noticed the drop in performance.
After closing BoincManager on one box running some MW WU's the crunch time dropped to 560 seconds, compared to around 770 previously I think (MW doesn't retain a record for very long). I'm waiting on other projects to complete fresh WU's to double check, but it seems to be a genuine drain on CPU.
In otherwords, this is a very long post to say: if you are running GPU grid on Linux, close the BoincManager Window!