Multi-timbrel issue.

[Zodiac247]

I am using

Imac
Logic Studio 8
Access Virus Ti desktop - firewire directly to the Imac
Presonus Firepod audio interface.

My issue is that whenever I try to run the desktop multitimbrel, and try to use more than one sound, or even sometimes just using ONE sound, the track will mud up.

For instance, I am playing an arpeggio, and a bass at the same time, and out of nowhere in the track a random note of the arp will hold and will drown out the rest of the Virus sounds as if it were frozen or a pad....

Any ideas why this is happening? The virus has 16 voices, and yet I can barely run 1!!!!

Please help me out on this.