You could rightly expect it to last longer under normal use, but it's *possibly* on its way out?

The only way to confirm that, is to try a different cable first of all. 

If a different cable doesn't help, try another monitor - or connect the system to your TV.

If the problem disappears, it confirms your Viewsonic is faulty.

If the same problem is displayed on a different monitor/TV, then obviously there's a problem elsewhere - possibly with your graphics card?