Maybe this has been covered but I did a quick search and didn't find anything.  First, I'm using X3d (or whatever the very latest version is), Windows 8.1 64bit.
 
I create a midi track and record some things to it.  If I use my Triton Rack or Roland JV1080 or even the Fantom synth in the Roland VS700, all is normal.  However, if I use ANY soft synth I get playback on only certain notes.  For example, I plug in Dimension Pro and choose finger bass and assign the midi track to it.  It only plays some of the notes and skips more than it plays- seemingly random.  Anyone encounter this? 

Try increasing the "Prepare Using nnn Milliseconds Buffers" setting in Preferences > MIDI > Playback and Recording to 500 or 750.

scook!  Thanks!  That seems to have done the gig!