Need help with cubase 6 and ezdrummer

Hello… I just got my self a new pc… and bought Cubase 6artist, with an CI2+.
But sometimes when i use Ezdrummer with it, it get that “crackling noise”- But its only on my guitar track, everthing else seems to run smooth. Im thinking that my pc maybe isnt fast enough- though it doesnt really seem to be working much (uses 35-50% of CPU)

Here some stats:

Windows home premium 64-bit
Amd Athlon II x2 - 215 dual-core processor
650Gb hard disc
6Gb DDr3 memory

Hope someone can help me. Im really new at this, and dont really know anything about pc´s, cubase or Ez Drummer

Thanks. Christoffer