I've not seen this before in smaller projects, but I have a large project and it is occurring every time I open this particular project: random tracks show as "flat-lined" for various audio clips.  Not all of them are gone, and they vary on each project opening.
 
I have been recomputing the picture cache, but it is annoying to have to do it every time.  I have plenty of space on the drive where the picture cache resides, so I am wondering if there is some "upper limit" for picture cache.  I thought there was but couldn't find it, but I suspect I am hitting a limit somewhere.

I performed a show recently and we recorded it onto 32 mono tracks that total 100 minutes of 48KHz/24-bit audio (split into six sections).  I've had large project files and not had an issue, though this one is about 50% longer than previous projects.

The size of the cache is set in AUD.ini variable PicCacheMB. The value may be changed in Preferences > Audio > Configuration File. IIRC, there is an upper limit to the value.

Delete old files in picture cache folder. 
SONAR does not make any kind of garbage collection there.

scook
The size of the cache is set in AUD.ini variable PicCacheMB. The value may be changed in Preferences > Audio > Configuration File. IIRC, there is an upper limit to the value.

Found it!  I knew there was a setting -- thanks :-)

panup
Delete old files in picture cache folder. 
SONAR does not make any kind of garbage collection there.

That's a good idea and it's been a while since I've done that (or rather, built a new PC and started fresh).  Holy moly, over 25,000 files in that folder!   I feel clean now