FINAL UPDATE: The problem in the end was caused by Voxengo's CurveEQ plug. Seems like if it's anywhere in a project it can cause issues, or possibly it has issues with Bitbridge since it's an older 32 bit plug.
 
It only happened in this project because I haven't used that plug in a long time, but I didn't suspect it because I used to use it a lot and I use a lot of Voxengo's other plugs all the time.

For the record:
 
I had exactly the same problem.
In my case I had to raise the midi playback buffers from 250ms to 500ms.
This in SONAR X2A.
 

Hey, I know this is an older thread but I thought I'd chime in...I'm on Win 7 64, running X1, and I found certain 32 bit plugs were the problem in my case for MIDI dropouts...I couldn't figure it for the longest time, as the project was only 8 or 9 tracks with a very few plugs, 1 instance of Session Drummer, and I cranked all my buffers way up (ASIO, MIDI, etc)...but finally when I deleted a couple of the 32 bit plugs I was using (the main culprit in my case was Lexicon Vintage Plate reverb), the problem was completely eliminated...so I'm assuming something to with BitBridge...I'm going to try jBridge and see if it works, cause I do really dig those Lex reverbs...but yeah, it seems like as long as you stick to 64 bit plugs, you're good to go.

They are posted somewhere, but there are a few plugins that are meant to be used POST-tracking, and massive dropouts can occur when they are inserted and you are still trying to record tracks.
 
Hitting the 'E' key on your computer keyboards toggles the Bypass All Effects function - so you can quickly determine if it is one or more of the plugins by simply hitting 'E' and seeing if that fixes it, then hitting 'E' again to turn all effects back on, then you can go through the process of looking at the plugins you DO use, and compare them to wherever that list of mixing/mastering plugs is.
 
If you ARE using one or more of the no-no plugs for recording, try swapping them out for less costly (look-ahead processing, CPU-intensive) plugs - just for recording purposes, and then swap to the mixing/mastering plugs once you are done with your initial tracking, and you should be fine.
 
 
Hope that helps, 
 
Bob Bone