NAS and multiple Workstations

I'm in an office with 3 Macs running FCP 7, with gigabit networking and a Seagate NAS 220 6TB in RAID 0. We had very good luck with sharing project files and media files using the NAS. Even complex projects with multiple codecs, multiclip edits, etc., played back very well.

Now, 6 months later, the NAS isn't able to keep up. The old projects still play just fine, but new projects skip and glitch in FCP. I can sustain 35 MB/s with a single file but once I start pulling multiple files, everything slows down. I've tried FCP settings, a different router, restarts, codecs, firmware, yada yada and I'm pretty sure it's just too much work for the poor NAS to handle. Does this make sense?

What are my other options for editing video from multiple workstations? Is there another NAS in the under $600 range that can keep up with video editing? Or should we just go back to local drives until we can afford to play with the big expensive hardware?