When you connected the notebook to the external display did you use the notebook as you normally would and the external screen stayed good throughout?
It is important that the notebook is used under exactly the same circumstances while hooked up to the external display as it is normally when your notebook screen goes bad.
Until you confirm the above I cannot say with 100& certainty that it is not the video chip but if the notebook was used under normal circumstances and the external screen stayed good then it would suggest that your video chip is ok and that the issue is either the MB to LCD screen cable is damaged or loose or it is the screen itself.
I always advise of the difficulties of repairing notebooks but you have done it before and so are well aware, so if the video cable needs checking out and the inside of the case cleaned out refer to chapter 3 onwards http://www.scribd.co...pire-6930-6930G
please disregard any other product or service offered at the link.
To help with cooling in the future consider using a notebook cooler such as the example @ http://www.newegg.co...N82E16834988075