Recently I've actually been having the same problems. This happens only if my monitor has been off and I turn it back on again.
The lines (there are two now on opposite sides of the monitor) usually go away if I try to bend (with my hands) the edges of the monitor. Yes it sounds crude, but it usually does the trick. I have a 24" Gateway monitor and I've been reading on line theres been some similar occurrences...so it seems we're not alone. Sometimes if I do nothing, it goes away, but most of the time it doesn't so I get frustrated. If you find a cause to this problem or even an easy solution, please let me know.