Onders - 8:15 am on Sep 14, 2011 (gmt 0)
Thanks for the responses - I'm hoping it's not something we haven't thought of yet, but it is probably something off the wall!
We have eliminated cookies and caching as possibilities - tested thoroughly.
With regards to caching the html / php output server-side, we do have some caching in place, but the randomness of the blank pages (all over the site and even through all the dynamic processes -i.e. search, results, description pages etc etc) seems to suggest it's not that
Will get the guys to look into this more though and post back here.