check DPC Latency (google it), make sure it’s not a "naughty" driver doing it, while you go down the list of devices to disable… to find out what/if that’s a problem…
try increasing buffer size and changing different numbers of buffers…
make sure no other process is running that ends up taking much CPU time over a long period. make sure no other process is running above Normal priority…
make sure the latest drivers are installed from your OEM provider (or the manufacturer if possible) for all of your hardware but especially mainboard, audio, video, networking, and usb/firewire
man… there’s like 100 things i could think of that could cause an instability…