If "tapping" the monitor "cured" the problem, I'd think the monitor is most likely to be the problem, or perhaps the cable/connector, close to the monitor. I wouldn't for example expext the graphics card in my tower case to be in any way influenced by a "normal blow" to the monitor. You may be able to get a better clue by jiggling the cable and seeing if that has any effect.

As grab said, there is no telling how long a monitor will last after developing such problems. I got around year from the last one I had that started acting up but I knew it could go at any time.