If hitting the monitor helps, there's definetely a connection problem somewhere. Some soldering points inside the monitor might have been broken.
In that kind of a situation I see no cheap and viable alternative to repair the monitor besides taking it into an AV repair shop. But that might as well cost more than the price of a new monitor.
This might be the perfect time to consider purchasing a new display unit

Oh, and I edited the topic title. Generally we recommend more informative titles without typos.