7800 mode diagonal noise/waves... is that normal?

My 7800 looks fine in 2600 mode, but when I play a 7800 title I get diagonal waves from the lower left of the screen to the upper right.  Its not unplayable, but noticable on games when the screen has a lot of black or solid color.  Is that normal or is something squirelly?  This system has been composite and svid modded, but I'm using the composite currently.

Thanks in advance guys and gals.