I have a very modern quad-core,Intel I7 PC with 16 GB of Ram running Windows 8 Pro 64bit , X3 64 bit, and a UA Apollo.   I have an old project I'm trying out in X3 (24 tracks of audio). I've used Audio Snap to create a Midi track of the Kick Drum that I've mapped to Steven Slate Drums.  Notes are randomly being dropped even though they are in the Midi Track.  I see the note being played by the midi Track (midi track meter bumps) but SSD isn't triggered.  This doesn't appear to be happening with any regularity. Sometimes it will drop 1 note, occasionally it'll drop 3 notes.  I've changed the buffers/latency up and down which doesn't appear to change anything.  I've checked for all kinds of other items such as hidden automation, hidden midi events (nothing but notes in the Event Viewer) and can find nothing.   
 
I just did the exact same thing with another project and had no drop outs at all. 
I can't figure out if this is an X3 problem or an SSD problem - though I use SSD with Ableton Live all the time and have never had a drop out.
I changed the sound set in SSD and the drop outs were less frequent but still occurred.
 
Disk is at 50%, Memory Usage is at 25%, CPUs look reasonable with nothing maxing out and all but the first are very low utilization. I can clearly see the first CPU jump when the kick plays but it doesn't max or stay there. 
 
I've played with the quality settings in SSD (drum kit detail, resampling quality, disk streaming) and it appears to change little.
 
Any suggestions for further trouble shooting?  Not making me a happy X3 customer.
 
 
 

What is Preferences > MIDI > Playback and Recording "Playback using nnn Millisecond buffers" set to? If it is set too low, notes get dropped. Try 500 or 750.

 Thanks that worked much better.  Only 1 kick has dropped with it set to 500.  But it's very strange as I have only 1 midi track playing single notes without any extra midi data - why do I need more playback buffers when the midi data stream is so light

There is nothing wrong with setting the value higher than 500. Older versions of SONAR used 500 as the default value. Also if you have MIDI data in measure one, you might want to insert a blank measure at the beginning of the project.

In general, I've found that Sonar's MIDI timing is excellent until you have actual audio recorded and start adding effects. Try disabling any effects on your audio tracks or busses. It could be that the timing is making a note sound like it dropped when in fact it was triggered at the wrong time.
 
Regards,
Dan
 

Ive had annoying playback dropouts with Addictive, and Superior, but I didn't know that the midi buffers were not the same as the audio buffers. Scook is the man!