i have not had hung notes like you describe, but one problem with C5K in Sonar is the mod wheel. it causes crackles in the audio.
sonar is the only host this occurs in for me. i have reported it to camel audio and they say the problem is in sonar. this seems confirmed by the fact that wrapping C5K in DirectiXer fixes the crackles.
i also reported it to cake several versions ago, but they haven't addressed it.