have you tried going in to sonars preferences and selecting your asio driver, click on the settings to open up your audio interface asio settings. try playing with the buffer size. you may be to low for sonar to handle. hense the drop outs.

Look for an option in the BIOS called HPET and disable it. Also, disable the onboard audio. My last dell laptop (Studio XPS 1640) did not like having both onboard audio and FW interface running. Shared too many resources. I also disable card readers when present.
 
Look for a BIOS update on the manufacturer's website as well.

guyshomenet
I also see no common problem source. Sometimes the disc load icon goes red, and sometimes it doesn't.

Sounds like you may be having a disk-streaming issue as opposed to CPU load or DPC spikes.
 
Check disk usage in Windows Resource Monitor during playback; if it's running high, try increasing your disk output (playback) buffer in Preferences.
 
If disk usage is high, and there aren't a lot of audio tracks in the project you may need to check disk performance independent of SONAR or look for other causes like concurrent virus-scanning.

Many thanks to everyone. Still working the problem (time is scarce this week) but here are some data points:
 
USB: Will try to move to the front-side buss, but right now it is the only live device on the back side apart from the Logitech wireless keyboard/mouse dongle.
 
SOUND DISABLE: Already tried disabling the monitor and MB-based sound. No change, though I may do this again.
 
ASIO BUFFERS: Currently playing with these. Might be helping a bit.
 
BIOS: There are a lot of options in BIOS which I don't have the documentation for. I did disable CIE, as per another thread. Can't tell if that had any effect at all. I do not see an HPET option. Still hunting for BIOS documentation (neither ASUS or AMI have responded to requests for docs).

We can close this one out. Unsure what magic combinations of tweaks I did made the difference, but I appear to have tamed the beast.