Random audio/fx/vst dropping on individual tracks

System:

Windows 10 Pro 64-bit
CPU
Intel Core i7 3770 @ 3.40GHz 41 °C
Ivy Bridge 22nm Technology
RAM
32,0Gt Dual-Channel DDR3 @ 823MHz (10-10-10-24)
Motherboard
ASUSTeK COMPUTER INC. P8Z77-V PRO/THUNDERBOLT (LGA1155) 32 °C
Graphics
NVIDIA GeForce GTX 970 (ASUStek Computer Inc)
Audio
KLARK TEKNIK DN32-USB

+Cubase 8.5.20

Problem:

Audio, FX- and VST-tracks are dropping individually, randomly. This happens about every 30 seconds and it lasts about a second only in some tracks at a time, not in every track. Not in specific place on timeline.

Problem is not hard drive drop. I have SSD hard drive and I have tried to back up project to another SSD drive and hdd. That did not help at all. I have my operating system and cubase on SSD drive and the project on another.

This problem concerns only one project. Other projects are fine.

This happens on playback but also when exporting and rendering. I tried to freeze tracks but did not help. I have 3rd party VST plugins and I have freezed them too.

One thing might help to detect problem, I noticed this happened on FX-tracks too. Long reverb dropped for a second. So it can’t be disc reading? It is some problem in cubase?

I tried also to change audio algoryth. That did not help. And of course I tried also to reduce latency but that is not the issue.

I also tried to change asio driver.

Does anyone know what is happening in my project? Thanks. -Aki