Is it as crazy as it sounds? The last safe set of data that Google made an impression of was back in May & June 2005 and that's why every time that things go wrong our chache dates revert to this earlier time. Not a problem if you haven't renamed your files since then, but if you have kept the same data and changed the page names ... you get a penalty for duplicate content that only exists in Google's cache bank.
It's the only reason that I can give for this constant drop in positions.
All the Best
Colin :-)