options/permalinks page causes error

When I create a htaccess file using admin/options/permalinks, a message says the htaccess file is not writable, so I have to cut and paste the code, save it as a file and upload. I do that by saving it as a.htaccess as a text file, upload, rename to .htaccess, then chmod to 666. It then worked ok.

I used it for a couple of days no problem. Then a few hours ago I was clicking around the options area (looking for the spam options actually). I clicked on the Permalink page, DID NOT DO ANYTHING, tried to go out of it then it returned a “INTERNAL SERVER ERROR 500” page.

I tried renaming the htaccess file to a.htaccess (the site became normal), then back to .htaccess (internal error again). I deleted it, setup a new a.htaccess via notepad process above, upload, rename, chmod 666 then it works again.

Whats going on? I dont wanna venture into the Options/Permalinks area for fear of it happening again.