Damaged cassette. VHS cassettes don't do well over repeated watchings. After a certain point the actual film in the cassettes get worn down.
Also, you might want to check out the VHS's head tracking. Most VHS's have a knob or set of buttons in which you have to manually tweak the 'VCR heads' so they can properly read the cassettes. Not much of a solution as for the most part, tracking can only clear up only so much of the static from overworn cassettes.