Trimmer, you`ve narrowed it down to a certain section and your absolutely correct that it is a bad solder joint, there are 2 things you can do, first get a bright light and a magnifying glass and go over the bottom of the board and look for it, should be fairly obvious with the magnifier, if not go here and order a can of "Freeze mist"
www.partstore.com this is a great "tool" for finding intermittents, it`s coolant in a spray can, have someone watch the screen while you spray in the area of the intermittent, when you hit the bad part the set will work, re-solder the part and any joints around it that look bad, end of problem, good luck with it, BMW Rider