You were right not to pay for the diagnosis as the repair out of warranty is more than a new 50 inch plasma. Next time consider Costco and warranties in the 10 year range.
PS. This issue, beat to death at http://forums.cnet.com/5208-6142_102-0.html?forumID=60&threadID=96534&start=0&tag=forum-w;forums06
I bought my Plasma, loved it. Now, early this year it started having what I'll call splotches, on the screen mostly with night scenes. Gateway was no help. When I call a repair company they wont give me any idea what the problem can be, they want to get the $200 initial fee. NO WAY! I want to know what is more than likely going on first of all, and then if it will be more economical to simply buy a new tv or repair this one. Also, will I have to do this every 3 1/2 years from now on or is this a fluke?