ston
I don't think adding more RAM will solve the problem.  It's a CPU core maxing out its processing capability.  Yours is more or less equivalent to an i7 running at a similar speed, so it's pretty damn good.  Try splitting the patches you want to play across two instances of Omnisphere, does that solve the problem? 

Just an update on this issue, if it could help someone else ---- I now upgraded my RAM from 8GB to 16GB and the problem has gone!   The other performance tips in this thread still help lower the spikes, but now the RAM has been increased I can actually play all those multis which were unplayable before, they now play perfectly.

I've been running Omnisphere here on just 3GB (!). It does amazingly well on limited RAM (better than Kontakt), but my experience has been that it gets very squirilly when it starts to run out of memory. Like weird distortion when fast-bouncing, and occasionally dropouts such as those described in the OP. No warnings like Kontakt gives you when RAM is low, and few tuning options for conserving memory.