Having played around with a few settings, it appears the answer to this was linked to the multicore settings of the virtual instrument. The default setting was off. Setting it to the max 4 cores for my CPU everything seems now to run OK. There is warning message which indicates if the DAW (Cubase 7.5) and the VSTi are both using multi core that dropouts could occur. I haven’t noticed this occurring on my iMac i5 processor (8gb ram).
If you are having loading VSTi this setting could be worth a look.
If any other users have observations on the multicore settings and ram/disk setting Id be interested in any feedback.
Cheers
David