I have just discovered I have this exact problem too since updating to 9.5.21 - all was fine prior to that. When ever the freeze happens, there is a massive spike in CPU usage (monitoring it with Stats menu) - it goes up to nearly 100% for all 4 cores then drops again. This happens every 2 to 3 seconds. Tried with both mp3 and wav export. It does it without any plugins enabled as well. I am on a Mac Pro with 10.13.4 installed - 3.7 GHz Quad-Core Intel Xeon E5 - 32GB RAM. Interface is a Saffire Scarlett 18i8.
I have very long mix downs that take up to ½ hour and would normally be able to work on other stuff with Cubase in the back ground, but this makes it impossible to do so.