Thanks to All Who Pitched In the Other Day

Several possibilities come immediately to mind. It would be helpful to know which version of Sonar you are using and your PC specs (this is why many of us put those details in our sigs, it makes troubleshooting easier as well as pointing to common setups that work well - or don't).

The first possibility is that the MIDI buffers are set too low in Sonar preferences. The default is 250milliseconds, but that often causes problems with MIDI playback getting scrambled. It's worth checking and if it's lower than 500ms try setting it to 500. Sometimes going even higher can be beneficial.

Another possibility is you are on the verge of an audio dropout. Dropouts can happen on even a lightly loaded PC because they're not always caused by high cpu or disc usage. Try increasing the ASIO buffer (assuming you have an interface that uses ASIO drivers) and see if that helps. If it does but the lag between playing and hearing the note played is a problem, then it may be time to do some troubleshooting to see if it can be lowered.

Often dropouts and similar behaviour are caused by PCI bus latency (not to be confused with the different kind of latency caused by high ASIO buffer settings) when other devices in the PC (or their drivers) interrupting data flow to do their audio-unrelated thing. Wireless network adaptors are a common culprit, so if you have one try temoving it and going into Windows control panel/device manager and disabling it. Reboot and see if the Sonar problem has gone away. There are freeware programmes that can monitor PCI bus latency and report if it's likely to cause problems. Which app to use depends in which version of Windows you have.

Can Sonar play back audio tracks without problems? Does disabling effects make things better or worse?