I think my problem was that I started with a V9 dwg file and just opened it up in V10 and saved and thought I could start working. I think the fill gets screwed up in the translation from V9 to V10 & can't save correctly to a dwg file. I don't know if your file was started in something other than V10. Hopefully everything will work wonderfully from now on ...... until it doesn't work again
hope it helps,
Michele