Are you sure it’s not a directory? 755 usually indicates it’s a directory, which needs to be eXecutable to enter. Or something that your site is running is changing it behind your back.
If it really is a file, try telnetting, or SSHing into your account and do a ‘chmod 644’ on the file.
By the way, I use Transmit, and I’ve never encountered permissions that didn’t stick.