While it's true that monitors with CRTs(picture tubes) are affected by electromagnetic interference, what you're experiencing doesn't sound like that.
I would suspect a failing capaciter in your horizontal sweep or power supply. Although, a failing/bad soldered connection in that same circuit can cause the same symptom. Try rapping firmly on the side, the problem may just go away. (bad soldered connections are a common problem in electronics and can appear or go away with no apparent reason)
Otherwise, it may need fixing.
BTW, by not having your monitor in front of me to physically check, I could be wrong.
regards,
Bookmarks