been a while since I used FCP2, but I remember that bug well.
Do you have Cleaner installed? There's an extension that'll mess up FCP renders. Dig around and look for the Radius DV encoder then trash it. It's an old DV codec so you won't miss it.
Then check your FCP preferences. Can't remember where it is in FCP2, but there's a checkbox for "alwas render in RGB" Make sure it's unchecked.
That should do ya. Upgrade to 4 when you can spare the cash - it's a whole world away from FCP2