[LAU] Pops and clicks but no xruns

Recently I posted regarding nice low latencies on my brand-spankin-new Thinkpad R60. I was a bit premature though, because when I actually run any sound through jack I'm still getting lots of pops and clicks in my audio. I am not, however, getting any Xruns. I'm not sure what to look for as a culprit. I already enabled "force 16 bit", which got rid of some nasty distortion, maybe something wacky with my IRQs? One of my worries is that the HDA card is on the same interrupt as my firewire and one of my USB drives. When I remove the modules though, leaving only HDA on its own interrupt, i still get the pops. When I use audio without jack I get no clicks or pops, and changing jackd to capture only doesn't seem to help. I also boosted the priority of that interrupt thread with chrt --fifo --pid 99 `pidof 'IRQ 4'` without any change. What could cause clicks without xruns? I'm baffled. -spencer