If it is a CRT-projection model, it is likely your focus block (the box with six dials on the front of your set). They develop dirt contamination or a burnt spot inside the dial for the red focus control. It's about a $35 part.
HOWEVER. An incredible amount of current flows through it and is not something you want to horse with whilst plugged-in. You will have a bad day.
Easiest fix is to pull up a program guide, pop the grill and pressboard cover off the front, and rock the red focus dial back and forth a few times. Watch the text portion of the screen to line up the focus.
HOWEVER. Don't touch the red Screen control dial. Cranking that up and down will result in a bad day - too much beam current will shut off the tv.