weird display affecting text and colors

Turned on the iMac today (iMac 5,2 model) and the display on the screen is bizarrely shifted in terms of the colors and text sharpness. The text has all turned grayish with an appearance like type produced by a worn out typewriter ribbon (faint and somewhat hollow in look). The colors have shifted radically from normal hues, the black has greatly faded. Same change in appearance is happening in every folder, document and picture on this iMac. This same screen has been suffering from a growing number of thin vertical lines (approximately fifty of them right now) which run from the top to bottom of the monitor screen, of various colors, which first appeared about a year ago and have been increasing in number. Let me add one last bizarre fact that serves to qualify this query. Here on the MacForums window, all of the same shifts in text and color are evident. But when I went to Apple support Communities, that website appeared normal. I typed in text there, which also was normal and legible: I then copied and pasted it into a Pages document to have a record of what I had written. That text pasted into the document with a perfectly normal appearance. The text in this message window is so blurred as to be nearly illegible. But when I clicked on Preview Post, the text there is normal. What could explain this behavior? Where should I look to trouble shoot that isn't plain and obvious? I noticed a little later than when the iMac came out of sleep, for less than a minute the screen appeared normal. Then it jumped back to this weird display.