I've started doing some MIDI work in Sonar after having mainly used it as a glorified tape recorder these past 15 years (a much loved glorified tape recorder). I put an instance of D16 Nepheton drum machine by adding instruments -> soft synths and a few extra MIDI tracks so I can keep my hi hat patterns on different channels to my kick snare patterns. The problem is when I loop a section while I work on the rhythm in that area after the first play through the timing slowly degenerates.

Does anyone else get this?
 
I've tried increasing the MIDI buffer ms to 1000. 
 
It's doing my head in, I can't groove.

I've seen other threads stating the same thing your talking about. I've always had fairly descent results looping in Sonar. But I really think its the nature of computer recording to have all these sorts of problems never ending. Dedicated systems built with a specific intention never seem to have these problems. For instance the Korg M3 sequencer works 99.9% of the time and midi in general seems much tighter not dropping notes and timing seems much tighter etc. Anyways, and I don't mean to get off topic, just mentioning my observations. Computer interfaces are great. But with all the advantages offered by computer recording, it seems we put up with so many problems. It's seems to be the nature of it.

On a more helpful note, I might try bypassing all effects in the project while loop recording to see if that helps.

It boggles my mind to think of how much more computers are powerful since I started computer recording around 2000 but yet the same problems persist today as back then.

CRX... what sound card are you using and what driver mode?
 
Factory cards can do this. It tends to show up more when working with MIDI based stuff using softsynths. Factory cards can generally handle the audio stuff pretty well.... aka: using it as a tape recorder..... but the load placed on the sound card chips to process the softsynths in real time is often too much.
 
If this is the problem, a dedicated USB interface running ASIO will generally solve the issue.....
 
Check your midi settings..... latency, and buffers first though..... you can generally adjust them a bit to get improved performance to a certain point.  The best solution though is a dedicated interface.

The MIDI timing problems while looping can usually be solved by engaging the metronome during playback. Just set the metronome to sound during playback in the settings, and put it on mute so you can't hear it. This will almost certainly solve your MIDI drift problem during looping. It's a weird workaround but works for most people.

Cheers, I'll try those tips. It's a Motu828 mk3 firewire card using the ASIO drivers and it's normally well behaved (has glitched out on me once or twice requiring me to quit SONAR and turn the card off and on again but for the most part has been a very good card).
 
I'd like to get MIDI sorted for softsynths, for my external gear I still mostly use my Atari (15 years solid use and still rock solid). When it's not looping the MIDI jitter in SONAR is pretty good compared to other DAWs (ahem cough Live is crap). If I could just get a bit more fluid in the PRV and sort the loop problem perhaps I'd go all in.

I find it to be an intermittent problem.  It's happened to me quite a few times before and drives me crazy when it does, but of late Sonar's been rock solid when looping.  No apparent rhyme or reason to it as far as I've been able to work out.  The last time it happened, I recreated the piece from scratch as a means to demonstrate it on the forums and the problem just went away.
 
I wasn't aware of the metronome work-around though, I'll bear it in mind if it happens again - ta!

Try look if sonar make correct loop range. Sometimes if i select area for loop, it dont select properly and selection is not accurate. This make timing inaccuracy when looping. Solution is look closer to loop points and set it correct manually. Maybe help.

MIDI FX can cause problems with looping. Bitbridge can, too, but I gather this is a native x64 VSTi...? It might help to understand exactly what you mean by "the timing slowly degenerates"; is the whole pattern just drifting progressively later or...? And does it happen with SessionDrummer or other VSTis?
 
I've only ever been able to reproduce a problem with loops not set to whole beat boundaries, and then it took on the order of 50 iterations for the sync error to become more than just a phase error against a rendered audio track, and a 100 to become really bothersome.

The loop boundaries are set to bar lines, no MIDI FX and it's the 64 bit version of the plug. What I mean by the timing slowly 'degenerates is' is the different channels (I have kick and snare on one, hats on another) seem to drift out of time with each other. It happens after the first loop and and gets steadily worse.