I agree Tim which is why I said that *I* had problems with Sonar 7 while others didn't seem to but Sonar 8 worked for *me* while others had problems.
The fact that overall he is having troubles with multiple versions of Sonar seems suspect to me.
I suspect his hardware or system configuration (ie:other running software, BIOS settings etc) are the main culprit here or at least a contributing factor.
I also agree with your statements regarding Studio One. In my opinion it's the one to beat at the moment for stability and performance of the audio engine and core components. I just hope they don't get caught up in feature bloat and lose focus on keeping the core program as solid as possible.
Getting back to the OP, if I were him, I would go get a bare drive, reset all BIOS settings to default, remove every card not needed including the second sound card, disable all not needed BIOS devices (ie:firewire ,floppy etc) and do a CLEAN install of Windows.
Then update Windows to latest SP2.
Update drivers for soundcard if needed.
THEN:::::::::::::
Do a fresh install of Sonar and see how it performs.
At least this way he can determine where the problem lies.
Sometimes a complete level set, start from square one and forget everything previously done, is necessary.
I know it's a lot of work but at this point IMHO for troubleshooting, so much has been done already, this is really the only way to properly document things.
Good luck!