I have reported this previously.
Basically Pro 13's rendering gets "damaged" if another operation is performed on the same PC which involves video playback and, I think, anything to do with the use of Direct-X.
I can reproduce this 100% by accessing the computer over remote desktop (RDP) or by playing a video with VLC or any other video player, while Pro 13 is rendering.
This video shows the issue: https://vimeo.com/177027490
about 15 seconds in. What this shows is that my 3xFX chain (contrast, colour correction, NewblueFX lens correction) is getting skipped every few frames. Clearly Pro is getting corrupted in some way. Once this corruption starts it never goes away until program restart.
Unfortunately there are some other apps which also trigger this issue. I have just had to abort a 100hr render... It may have been Photoshop (CS2)... or it may have been a video playback on any website, so even a visit to say facebook will do it.
MSP11 and MSP12 never had any of this.
I think the way to try to reproduce this is to render with an FX chain a bit like the above and play something with VLC at the same time.
Machine: win7-64, 24GB, i7-970 3GHz, GTX750 Kalm 2GB.
I would upgrade to Pro 14 if it fixes this, because it is costing me a lot of time.