I had problems with the touchscreen with my N900 as well. One day I could no longer put my pin in the lockscreen. In my case it was most probably moisure in the contacts of the digitizer, since the solution turned out to be: disassemble the digitizer, clean the contacts, and assemble back. The really hard part for me was taking the tiny screws out :-( So if you are attempting that, first go find some good A00 tools and search for the N900 service manual in this forum.