listen
I (in error) said increase your buffer size - when I should have said decrease the buffer size and see if that solves the problem (that is while your recording).  After you have completed your recordings then bring your buffer size back up to where you usually have it...

Thanks for clarifying this. Unfortunately decreasing the buffer size (even to it's minimum) didn't resolve the issue either.

Well I contacted Cakewalk support about this and after 2 months, still had no reply. This isn't the first time I've contacted support and got no response. 

http://forum.cakewalk.com/Sonar-issues-with-MP4-video-files-Please-help-2016-m3347825.aspx
 
You are expecting a fix when there isn't one available. What can Cake say, except "You're right, we know"? Several SONAR users use it professionally with video every day because they have learned to work around it's inherent limitations - i.e., convert to AVI or to WMV, or create the basic score in SONAR, then cut and edit to video in Vegas or something else.
If you are committed to using SONAR for doing what you need, I recommend searching the forum and tracking down users who have worked around your issue - it's come up many times. If not, Cakewalk seems to encourage users to use multiple tools to get the job done - not just with video. It used to annoy but I've made my piece with it (LOL) -  (and extended SPlat for another year). 
 

This is a known bug and it isn't fixed, yet. (CWBRN34262)
http://forum.cakewalk.com/CWBRN34262-Sonar-X3-Video-trouble-during-countin-m3172255.aspx#3172255
Other bug may have a higher priority.

finalymetten
This is a known bug and it isn't fixed, yet. (CWBRN34262)
http://forum.cakewalk.com/CWBRN34262-Sonar-X3-Video-trouble-during-countin-m3172255.aspx#3172255
Other bug may have a higher priority.

Thanks for this, much appreciated. As Cakewalk never responded to my support request, I had no idea this was an actual bug (as opposed to me missing something) so that's good to know it's not just me! Not sure it's the exact same issue as mine but certainly sounds like it could be related - seems like Cakewalk has issues wiih MP4s at least.
Do you know if I am able to track the progress of this issue?

I have no idea if you can track the problem report from sombody else. Maybe one of the form hosts can answer this question. I thought it is possible because there are so much CWBRNs shared in the forum.
 
The status is “MoreInformationNeeded”. I sent them this demo project to illustrate the bug. I thought it is enough information. If I have some spare time I will add a video to show the bug.
 
If you think your issue is different you can fill out a problem report as well, but if you are still using X3 I don’t know how handle bug reports of version which will no longer be developed.