Hi christian,
It sounds as if it is a board problem. could be the touch controller ic. the picture i have linked shows whats under the little metal cover on the logic board behind the touch connector. i would suggest having a look at those components and see if anything is out of the ordinary and clean with some ipa.
http://m.eet.com/media/1201343/Broadcom-touch-screen-controllers.jpg
hope this helps you in some way