Something wrong with 7.5.4. … 
Stuck on midi notes ( EWQL Hollywood Strings Gold), audio drops sometimes…
7.5.3 was quite good. I know , I have to buy 8 and forget 
Roll back to 7.5.3 solved all problems. I’m affraid about Cubase 8 Pro upgrade
I have RME Fireface UC, Win764 on SSD, 64GB RAM ( Kingston HyperX 2400 )