|Make sure there are no bent pins on the connector on the end of it's video cable, that the connector is plugged all the way into it's port, and that the connector is ~ 90 degrees coming out of (perpendicular to) the port. |
Other than possibly that, random lines or artifacts are almost always caused by a hardware problem rather than a software problem.
Its NOT a good idea to buy ANY used LCD monitor, unless you're SURE it has not been used much. The display of an LCD monitor on average has a much shorter useful life than CRT monitors do / did. The better the brand's reputation and model, the more likely the LCD display will last longer.
"...1 year warranty..." A Westinghouse model is probably one of the el-cheapos.