cubase 5-1 crackle

hello, I am using cubase 5-1 and have m-audio audiophile and delta66 pci soundcards which I recently put into a custom built computer asrock z87 motherboard, 8gb RAM, running windows 7 SP1. hooked up to a stereo amplifier but no matter what the audio output is sent to I get noise during playback of my music the sound deteriorates and crackles after a short time, until I click reset in the vst audio system page which brings the sound clear again for a while. i didnt have this problem on a previous dell computer using same operating system, version of cubase and soundcards.
would appreciate it if can anyone shed some light on why this might happen and what I could do about it.