Thanks for all your advice so far. With your input plus from another board I have established what the problem was (although all a black box to me).
I wasn't aware that on CPUz you could see the PCIE link ratio (not that I understand what it is
). Anyway it turns out when I have the fps problem the ratio goes from 16x to 1x.
So I checked three things... CPU volts, MCH volts and PCIE freq. all against a 9x333 OC.
Stock volts and 100 PCIE link = 1x link
+0.1V MCH = 16x link
102 PCIE freq = 16x link
CPU volts 1.3 (up 0.05) = 1x link
So great... now have a specific test for probelm but 2 solns. Any advice on which is best (for long term stability/lowest 'wear' etc?).