Dreamshell image problem

[Johny45]

Hello
I am new in the dreamcast world but i really wanted to get one and i finally did it. But now i have a problem with the dreamshell display image on the tv when i try to play.

The image when i boot up the program it pushes down the display image on the tv leaving the top black. It like cut half the image turning the top black and not a complete image on rest down of the tv.

Did anyone have this problem?
Can you help me?
(only after on the menu on the dreamshell the image suffers the problem, in the beginning no problem)

Hope you can respond me.

TY