Vaio VGN-TX47SP/B screen goes dark

Hello all. I am having a heck of a time with my laptop and hoping that someone on these threads can help me out. Here's the quick rundown: I bought my Sony Vaio slimline VGN-TX47SP/B laptop about 2 years ago (just long enough that the warranty has run out) from a Sony store in Bangkok. The laptop was issued with XP Pro, but said it was Vista ready. I upgraded to Vista Ultimate in February of this year and everything worked great for some time. After a few weeks, I had the problem in that, whenever I unplugged the AC cord, the screen would stay bright for about 15 seconds and then go completely black (I could make out the screen if I shine light on it). Once I plug the AC back in, after about 10 seconds, the screen comes back on bright. I took the laptop into Sony Repair here in Honolulu for maintenance. I got a call the next day saying it was software related and I needed to do a system restore. I did that and the same problem occurred. Sony tested the LCD and screen inverter and said they were both good.

External monitor worked fine when attached. When I let the battery drain down, just before it is empty, all of a sudden the screen lights up bright as can be - and then 10-15 seconds later, the battery is completely dead. I recently replaced the battery, so I believe I have ruled that out as the culprit.

All of the power settings have been adjusted properly - Sony checked them out and ruled out a settings issue. It is not a question of being able to hit FN-F7 or anything like that. The screen goes dark on battery and nothing I've done except to shut down and restart will make it bright again. When I start the computer on battery power, it will stay bright for about 15 seconds then goes dark.

Anyone have any ideas? I am now at a complete loss as to what the problem can be...help!!!!