On my 65VT50, I find that from a cold start, the picture appears a little lighter and after about an hour is correct based on how I have it calibrated. This is the opposite of what you're experiencing which is puzzling.
So that you can confirm what you are experiencing, you need to use a pluge pattern which you can download free from AVS. Here's a good one. http://www.avsforum.com/t/948496/avs-hd-709-blu-ray-mp4-calibration
The point here is that you need to use the pluge pattern when it looks right to your eye and also after it's doing it's light black routine so that you can be sure that it's not changing just because of a combination of AGC(automatic gain control) and APL (average picture level which is just how bright the picture is).
You need to move fast on this as you sure don't want to let the warranty run out if it turns out to be hardware related.
Out of curiousity, is your panel wall mounted or on a stand? Room kept cool?
Samsung 64F8500, Panasonic 65VT50, Oppo 95, Tivo Roamio for OTA, Dish VIP722, Marantz AV8801 preamp, Rotel Amps, Atlantic Tech 8200 speakers, Seaton Submersive HP, Calman 5, Chromapure, Accupel DVG-5000, VideoForge HDMI II, i1Display3pro, i1pro2, eecolor colorbox.