Performance CPU meters spiking

I have recently had fairly regular issues with the performance CPU meters spiking irregularly, along with audible pops/clicks.   I ran DPC latency check and it seems all ok, nothing wrong there - even when the spikes show in Sonar.   Current latency always well under 100us.
 
My PC is reasonably powerful (see signature), and I don't think I'm giving it too much work to do - mainly soft synths, perhaps up to 10 instances open, not all playing.
 
Any ideas what is most likely thing to check next?

A lot depends on the soft synths. Some suck a lot more power than others. Also, what is your sample rate and number of sample buffers?

May be telling my granny how to suck eggs here..... but if I'm doing anything audio/daw related I turn off my network adapter as well as the various shields that AVG uses as well as any un-needed processes that are running thereby ensuring that if a problem occurs its not network/internet related.
 
I never boter when doing midi/daw related things as midi data is smaller than audio so my head tells me this shouldnt be an issue

Thanks for the input here, really appreciated.
 
The soft synth I had most issues with (and it can happen even when its the one and only track and soft synth in the project) is U-He's Diva. Using that on some patches causes the furst yellow bar on my CPU performance metre to spike in the red. There is a multicore button in Diva patches, and when turned ON that made things work a lot better (no spikes so far) - (I don't know what that little button does but it works).
 
I do habitually have internet ON, whilst working with Sonar, and recently noticed that does noticably have an adverse affect on performance, so I now disable the connection each time I work with Sonar.
 
So far those 2 things alone have helped a lot, although on some projects the first yellow bar looks like its about to peak into the red on occasions, but so far has not.
 
As for power - I have one plug to the wall into a adapter with 8 earthed sockets, so everything is going into one power source, PC, Midi controller, monitors etc.
 
Re the ASIO buffers, doing some tests I see 64 causes regualr spikes, 1024 has noticable latency, and 256 seems to be the right balance for my system between usability and perfomance.
 
Re Windows, I have Windows 7, set for "Best Performance" but other than that I have done no special tweaks.

To confirm scook - yes, Diva is known to require a lot of CPU power.

Ah yes, the accuracy setting, is working OK with setting at "Great", and I think even "Fast" sounds quite OK for tracking purposes (and is noticably less CPU whn at this setting), and when bounced will be at highest quality.