This is not surprising with only two disks in a raid 1. Your disk setup is likely causing this problem, not enough bandwidth. Everything on the same disk (OS, programs, pagefile, scratch, media, renders, temp files, etc. AND having to mirror that to another disk, likely on a IHCR chip) is probably the cause.
Look here for how to setup your disks, especially about storage:
Your disk setup is likely causing this problem
I disagree. Throughput of a disk subsystem should not affect the content of the final export. The speed of such, maybe, but not the content.
I would agree that disk speed would affect real time playback within Premiere, but I'm not sure it would necessarly affect playback after video has been rendered and exported. Real time playback within Premiere is fine, i'm only expierencing this problem while playing back an exporting movie file (.mov or .flv./f4v)
What happens when you export to MS DV AVI type2?