A little desynchronization while project playback in Cubase 5

I did a little piece and looped it in Cubase 5. 5 midi tracks sounded by Virus TI2 desktop and 4 tracks sounded by vsti.
Virus connected by usb. One virus patch has an arpeggiator. So when i start playback in cycle i can hear virus sounds often play with a little desynchronization with Vsti. Why ? How to fix this ?

Yes, Marc in this case it seems syncs more better. I tried to extend R-L (first of all it concerns the Left) loop locators behind the midi parts, but i don't understand why sync is bad when the locators are on the borders (beginning and finish) of midi parts ?

Ok, you gave me an idea to test virus cycle playback in another DAW's like Sonar and Ableton When i make this i will write about results here.