Vizio 32 shaded darker area from middle of screen toward left edg

I purchased a Vizio 32 from Costco two years ago and have been very pleased with the picture and performance. I am currently having an intermittent problem with the picture. I still have a picture and sound, but have noticed a shaded darker area from about the middle of the screen towards the left edge. It seems to come and go. I am discouraged after reading all the comments about Vizio tv's, and sorry I purchased one.