Sound cuts & multitrack

[Acidophilius]

Hello all,

I'm a new owner of a Virus TI Desktop. I sequence it with Ableton Live 6 (which worked also with my Korg EMX1+ER1MK2).

The virus is plugged by USB port, the software is installed and works good (all midi signals are sent and received).

My problem is the following :
When i play with more than 8 tracks on Ableton, some sounds are usually cut which is really boring. I've tried to cut all effects (delay reverbs in 1st) and all polyphonies and automations after. The result is better but there are always some quick cuts.
I think this problem can be caused by sounds which ask lot of virus ressources but i would like to have more precisions about that. Isn't it a midi trouble?

Do you know this problem when you write your tracks and how do you do when this problem happens?

Thank you for your help.