I am wondering if there might be some background task that starts itself once in a while, and that happens to spike DPC latency to the point where you get into lag issues with Dimension Pro.
 
Can you check for scheduled tasks?
 
Also, if you happen to be running with a Wi-Fi adapter, these are notorious for spiking DPC latency.  IF you have one present, try looking for a hardware switch or function key to shut it off, just prior to launching Sonar.  If neither are present, then go into Windows Device Manager, find your Wi-Fi adapter, and disable it.  (not uninstall), again just prior to launching Sonar.
 
When you are done with your Sonar session and close Sonar, go back and enable your Wi-Fi adapter again, and all should be fine again.
 
And, since you are running Windows 7 (as opposed to 8), the DPC Latency Checker program can help you determine if your system has some sort of spiking issue that would intermittently cause issues with Sonar.
 
Here is the download (you just run it while Sonar is not running, to let it measure your basic system DPC latency situation, which it scrolls for display):
 
http://www.thesycon.de/eng/latency_check.shtml
 
Bob Bone
 

Thanks for the help Robert but 1) My audio computer is never on line. No WiFi adapters. This rig is used strictly for recording only. I only go online with it to get the necessary updates for windows and drivers. 2) I've had the computer for about 10 months and this is the first time this has happened. I usually run marathon sessions without any problems at all. Weird thing is the problem I was having seemed to vanish into thin air. (Knocking on wood here!) I'm really at a loss here but as long as things stay like they are I'm a happy guy. Lol.

In all versions from  PE4 on I've experienced situations (mostly with amp sims) where a huge latency suddenly occurs (usually after finishing a take and auditioning), and toggling audio engine on and off makes it go away.

If I forget to disable the windows backup before I start Sonar I can work for an hour or so then Sonar suddenly grinds to a halt...

In my case it's just Windows using all the HDD bandwidth looking for things to backup... There are no bugs involved anywhere.

Karyn
If I forget to disable the windows backup before I start Sonar I can work for an hour or so then Sonar suddenly grinds to a halt...

In my case it's just Windows using all the HDD bandwidth looking for things to backup... There are no bugs involved anywhere.

Hmmmm. I think you might have something here. I did see some new files that were added to my backup disk.