Have I accidentally hit a key that's made this happen? The PRV has always scrolled - not something I've had a problem with before. It's just stopped - I have manually move things along using the bottom slider. ??? 
 
Edit: Saved project, closed Sonar and re-opened, all OK. Weird....

you can toggle between scrolling and not scrolling with the scroll lock key :)

It will also (by default) stop scrolling if you click within it to edit.   Right-clicking in the PRV timeline should get it going again.     Or pressing the Scroll Lock key.