need some help --> Virus TI patch problem..

[P1-2k]

eay guys,

I building a track with my virus ti on windows XP. I bumping into a big problem with my ti. I've build a melody for my track with just 2() patches on different midichannels.Now here comes the problem:
when i playback the melody, some of the notes fall away and won't play (if i play both midichannels at the same time). But when i mute one of the patches, the melody sounds like it should be and plays all the notes.

It's seems like the sounds are conflicting with eachother but i'm wondering why since i'm only using 2 patches

I was wondering if anybody has the same problem and maybe found a solution?

some sideinfo:
- windows xp SP2
- Virus OS 2.5
- soundcard: m-audio delta 2496
- processor: core2duo
- Cubase sx3
- using USB to connect virus to computer

looking forward for some replys!
tnx!

Frank