SnigRO,
When selecting folders try setting Repeat to Normal and select the folder by pressing play. This helps on most players.
For full screen video what size is your screen 1.5 or 1.8?
Trying to upgrade your firmware is not recommended as you could end up killing your player completely.
If you have not done it yet I suggest you backup your firmware using s1fwx which you can download
HERE and a guide on how to do this is
HERE
If you have any problems with your player in the future the backup can be very helpful.
If you get a chance it would be great if you could upload your firmware to site by using the upload button at the top of the page
HERE
I know there are a few members looking for firmware for your type of player so it would be a great help to them.
Let me know what size your LCD screen is and I will help you with the fullscreen video.
Thanks