Horizontal or vertical hold not working? Magnavox wavy lines?

I have a 25" Magnavox 25TRC1 0101, about 7 years old, that just started to display "swimming" lines across most channels. Audio is fine and channels tune to correct station. Worked fine yesterday, with no indication something was beginning to go wrong. Can't tell if they are mostly horizontal or vertical lines at this point. Some channels tune in clearly, then begin to swim a bit, others alternate between clear and swimming constantly, while others always swim and are never clear. Same reception problem when taken off cable. Have hooked up another TV to the cable outlet and it works fine, so it must be this set. With no external control buttons for H/V, what do I look for when I go inside to see if an H/V adjustment will fix this problem? Have never been inside a TV before, but if this is a straight-forward fix, would like to attempt myself before calling for a repair.