While playing/recording using Softsynths a note will play out of nowhere. I have to stop the project find the note it's playing, hit the note to make it stop. Can't use any softsynths to complete projects. Does anyone have a clue how to stop this or what's causing it? I downloaded X2a, but still having major problems.

Which soft synth(s) is this happening with?

Is it a "stuck" note that is part of your project but won't switch off?

If it is, make sure you leave a tiny gap between this note and the next one.
This has helped me out in the past.

A description of the computer, the operating system,  and sound card as well as the synth doing this would be helpful.

If you look at the midi track in either PRV or Staff View, you should see all the notes being sent to the synth. 

Every one of them should play based on their note value, and on the flip side, nothing should play that is NOT displayed in either view. 

mysterious notes that "just pop up" in the sound stream are coming from somewhere. It could be related to the soundcard if it is happening often. 

Thanks so far... the PC is a custom built by iBuyPower, windows 8 OS. have no idea about sound card. It has a MSI board AMD 3.6 processor. It has a video card, but don't us it, or know anything about it. The problem happens with my old controller and new controller. No matter what key I'm playing in, the softsynth will play a note on it's own that I didn't play and lock it down until I find the note on the controller and play it to make it shut off. It does it with any string, organ, horn type of sound in any setting-- softshyth- cakewalk sounds, demensons LE. To be able to use the controller, I have to unplug the interface turn on Sonar, wait for a popup that says Midi source unplugged..plug in the interface and hit OK on a popup that says Do you want Sonar to add this device. I've used Sonar for many years and just up graded to X2. Playing strings and symphony on tracks leaving spaces between notes is not an option. Sonar 7 never had that problem. Thanks....Paea

 The note just comes out of nowhere by itself no matter what key I'm playing in, I have to hit that note on the controller to make it stop playing..

I am using a borrowed Alesis i02 interface, my old one wouldn't work, I have a Tascam 800 coming.. could that be the problem? ... A friend just helped me... It says the sound card is Realteck High deffinition audio.

I have seen all sorts of issues with the factory sound cards while helping folks in the MC forum get started. 

notes out of nowhere, missing notes, and hung notes. At times it has to do with the buffers and the way the soundcard handles or in many cases, doesn't handle the notes.  .....with the realtek, normally you want to use the MME driver. Some folks have used ASIO4ALL with some degree of success. You can try that also as a last resort before buying a new and better card/interface.  

OK so try this. 

You say the issue is mysterious notes coming in the audio stream that do not appear in the staff or PRV and hang up until forced to stop.

So..... select the track involved and freeze the synth or bounce it to audio. This is an "off-line" process that takes the synth output and creates a new audio track from the synth's midi data. 

Since the program is only doing that one thing, and not trying to do it in real time, it will take as long as it needs to render the track accurately to audio. If you have frozen the track simply click play when it's done. If you bounced it be sure to mute the original before you click play. 

You should not hear the mystery note in the new track. But the whole point is to see if you do. If you hear it, you need to look for that note because it is there somewhere. If it's not there, as I suspect it will not be, the sound card is likely causing strange things to occur due to it's inability to function in the high demand environment of the Sonar world.  The option is a better sound card or bounce the synths to audio as a work around.  

Thanks Gh... I'm using AISO to run the Alesis, but comments have been pointing to the sound card. My new Tascam should be in today or Monday. I never had this problem with the old Tascam US122. I couldn't get the new PC to accept the 122 drivers. Thanks... let you know what happens with the new interface. The Tascam US800 didn't come with driver DVD. Down loaded drivers took about 5 hours to install. Got everything installed everything worked fine, no problem with softsynth sticking...But... I shut the PC down for a while, went back, the software or the PC lost the drivers, nothing... uninstalled, re-installed drivers, couple of hours had things working again, except for the controller. interface played sound Sonar would record, but when I kept trying to use the controller the pop up kept saying USB device not recognized... just don't know

Got rid of the windows 8 PC. My friend that's a PC tech told me w-8 is not software to machine friendly. Got PC with W-7. When I could get the software to recognize the new interface the note problem didn't happen, but when I turned off the software it crashed my PC, when I got it back on all of the drivers had been changed back to the drivers on windows, the interface drivers were gone.