Specifically when playing a midi track through a soft synth there is a 30msec (80 ticks at my tempo) window in which midi notes are lost which occurs about every 4 seconds. I have reproduced this problem using just 2 notes 4 seconds apart so as to rule out overloading. Using a hardware synth is fine.
I am using Sonar X1d.
I hope that this is just a setting that needs changing but I haven't found one yet that makes any difference???
 
Thanks
 

Sorry if this has no relevance, but isn't Sonar X1e the latest update to X1?
 
(could be one of the reasons??? IDK)

 
X1d was the last update.
 
Back around then, some similar bugs were worked around by turning the metronome on, and muting its buss if you didn't need the metronome.  This solved my midi notes disappearing.  I had a simple loop in which notes would fall off on each iteration until there were none left.

Try increasing your midi buffers from 250 to 500

+1 they changed the default for some reason a release or 2 back, and lots of folks have found 250 to be too small.
 
Bob Bone
 

 
The metronome didn't make any difference (audio or midi metronomes).
 
The Midi play back buffer was set to 32msec. Increasing this to 100msec actually made things far worse, but 200msec+ cleared the problem in this case.
 
Thank you for your help.

 
... thinking about the logic. 32msec was about the length of the loss window, but is quite short compared to the note spacing. So I reckon the buffer with emptying and rebuffering more than every 4 seconds but the other cases fell within gaps between notes. With 100msec the loss window is wider hence worse. With 200msec+ the buffer never emptied.
The only doubt I still have though, is there a down side to a much larger buffer?
 

I run mine with a 2000 setting to compensate for a humanizing mfx.
If anything, it may take up more memory but I've never run into an issue as a result.
 

The Pre-X1 default was 500ms for years. X1 lowered it to 250 which caused problems for a lot of systems. What works best is very system-dependent but does not necessarily correlate to system horsepower. You might go a long time at any given setting with no problem, and then find that one particular part of one particular track in one particular project at one particular tempo results in dropped notes, and you have to bump it up a little.
 
I've found the old default of 500 works great 99.9% of the time. Every once in a while, I'll notice a dropped note, and have to bump it up to 550 or 600 for that one project. Sometimes just moving the note(s) a single tick one way or the other will fix it. I have a sense that it might happen more often with hard-quantized MIDI where there can be a lot going on right on the beat with multiple synths and drum parts hitting at the same time. I think it helps to record stuff in real time and not over-quantize it.
 
MIDI to Hardware outputs isn't buffered so it isn't affected. Really what's being buffered is not MIDI per se, but pre-rendered audio from soft synths. For this reason, I'd stay away from extremely high settings such as Gustabo suggested uses. This can unecessarily delay the start of playback, and can have other undesirable side-effects such as glitches when looping or moving the Now time during playback. For the record, I was unable to reproduce Gustabo's issues with MFX at lower buffer settings.
 
 

brundlefly
I'd stay away from extremely high settings such as Gustabo suggested,

Wasn't a suggestion, just stated what I'm running.