I am suddenly experiencing drastic latency between Sonar and my midi device. In the fast this was solved by changing the block size (64, 128, 256, 512) in the Presonus (USB) software. I believe I have had it at 256 up to now, but suddenly the latency is back and I've tried changing the settings, rebooting, and I've looked for settings in Sonar but nothing has worked. Any suggestions?

Is this latency occurring on a brand new, empty project, or one with plugins already on the tracks? If it has plugins, enable FX bypass and see if that helps. If it does, you have a plugin causing the latency. Find it and bypass it until you are ready to start mixing.
 
Regards,
Dan
 

That does seem to be the issue, thanks