Yep. Definitely a monitor issue.
Make sure your resolution and refrsh rates are not exceeding the monitor's specs. The most common mistake people make is assuming you can use the max resolution and the max refesh rate together. This causes overheating and damage to the monitor, leaving permanent defects in the picture or burning the monitor out completely. Check your manufacturer's manual for the recommended refresh rates to use with a given resolution.
It could be you've got a loose connection there or it may just be the monitor is either aging or defective ( you did not mention how old this monitor is or if it is CRT or LCD ).
Those types of image defects are not uncommon with older LCD monitors. If it is a CRT, then something has gone wacked with the vertical deflection and you're getting some parallax in the scan lines.
Hope this helps narrow it down for you.
