Following up to myself (again) the rest of this story belongs in the "no duh" category. i found that cdparanoia was niced down to 19 on my desktop machine while on the laptop it was set to 0 (using grip, which allows you to set a nice value for the ripper and encoder). i think this may have been the problem all along - basically unless cdparanoia is throttled a little bit it just swamps the cpu with cdrom dma interrupts.
setting the laptop to nice cdparanoia to 19 helps. then setting the umaskirq flag makes the machine usable during cd ripping. rob