Hi everybody,
I have a big problem with my virus. So I use Cubase 7/8 (same problem on both) and never had this problem before. So I open VST instruments, active OUT 1 & 2 and create two associated midi channels. i create my melodies put my presets and then ... problem that appeared last night : when I play both layers, the 2/4 of each bar don't play for one layer. when I solo the synth that doesn't play , nothing wrong it plays every note, but when I play them together again.. meh. i thought it was a voicing problem so I change every preset and put an init... same problem so it isn't. I tried to update the drivers, do a manual reset on the hardware stuff changed the usb... nothing work. I don't know from where it can come .... Help 
thanks for you help !