Get a small fan and point it at the back of the TV and see if it gets better.
Now no offense intended but if you thought it was a heat sink issue, what stopped you from cleaning or working that idea?
while watching my TV the other day, it just stopped working. The TV was on but it didn't have sound or a picture. after cutting it off for a little while, I cut it back on and the sound and picture was back. But after about 10 mins, it cut off again. Its like if it gets hot, it cuts off. I was thinking that is was a heat sync problem. What does any one else think/