Been working today and noticed some VSTs (RapPro so far) are giving me sputtering latency and dropouts when try and record. I thought it was everything but I loaded up NI's FM8 and it worked fine. 
 
I've got DPC Latency Checker up and it's showing me green goodness the whole time. I'm running my Octa Capture at a 256 buffer size. I'm gonna go load up a few more contenders and see what's what, but figured I'd chime in first in case anyone had good troubleshooting tips.
 

I'm gonna give this a bump to see if I catch any of the tech gurus on here. :-)

Another Roland interface dropping out? 
http://forum.cakewalk.com/Audio-Dropouts-The-engine-has-been-stopped-unexpectedly-m3656635.aspx
 

This?  http://forum.cakewalk.com/Audio-Dropouts-The-engine-has-been-stopped-unexpectedly-m3656635.aspx
 
Solution?  The user replaced his Roland quad capture with a new audio interface.

Okee. Well after those depressing reads I realized I DO have a Focusrite SOLO sitting here from my laptop rig so I swapped that in instead. I'm running at 44.1/16 and have tried 256 and 512 for my buffers. 
 
RapPro is still sputtering and laggy. FM8 (these examples are arbitrary at this point) is okay, records okay too.
 
DPC Latency is still in the green but right up against the 500microsec mark.
 
So, same result with different interface. :-(
 

DID you increase your midi buffers? I use 1,200 

Does that happens with any patch in the RP? Check RAM/CPU consumption on the system (with Windows resource monitor).
Note that DPC is only a part of the story, f.e. I have dropouts when recording (only) audio (only) under low settings (in Sonar only), while DPC shows everything is "green". I mean I can record MIDI at that settings in Sonar, I can just play with echo monitoring on (without recording) in Sonar and I can record in Waveform. But under the same conditions, I have to increase the buffer to record in Sonar (my current guess that is something to do with graphics )

MIDI buffers: increased (I'm assuming the main ones, not the playback ones listed further down in options)
Still there, only with more latency now?
 
Certain patches in RapPro DO play a little better, but what can that be a horsepower issue? I'd think my rig could handle it. It WAS handling it a few months back.
 
Gonna try cycling through plugs and make a rough list, but my gut tells me the problem is somewhere else. 
Plus, with all the buffer (audio and MIDI) and driver settings tweaks my latency is pretty high now. I had better latency on my old Firebox! (I loved that unit)
 

My experiance is that setting midi buffers high has zero effect on midi latency. What it does is allow Sonar to store more data in it's buffers so if there is a LOT of data it will not get missed causeing a midi drop out. 
As far as I can tell the higher the number the better. Not sure what the perfect number is but I just cranked it to 1,200 and I have no problems. 800 might also be fine. But the default,  250 is not enough for most of us

DPC Latency Checker does not support Windows 10 yet.  Windows 7 and lower only...
 
Try Replendence LatencyMon instead.  http://www.resplendence.com/latencymon