Pops and clicks

Hi there,
I recently upgraded my audio PC from Windows XP to Windows 7 and simultaniously my M-Audio ProFire2626 to the Steinberg MR816. I’m experiencing pops, clicks and drop-outs (especially with MIDI tracks) while my CPU meter isn’t showing any peaks. I already upgraded from 2 to 4 gB RAM without effect. I’m using a firewire card with TI chipset. I use the Roland UM-ONE as a connector between my MIDI keyboard and my PC.

Intel Core 2 Quad Q6600 (S775, 4x 2.4GHz, 8MB, 1066MHz FSB, Boxed) (G0 stepping)
Asus P5K64 WS, P35 (ATX, PCI-e, Sound, LAN, SATA II, RAID, 1394)
Corsair 4GB DDR3 (2x2GB) PC10600 1333Mhz CL9 (TWIN3X2048-1333C9)
NEC AD-7200S 20x12xRAM
Asus GeForce 8500 GT 512MB DDR2

Any ideas on what could cause this problem? Everything worked fine with XP and the ProFire2626…