Success!!
I completely took it apart again. Removed the display assembly (but not separating this), mother board and sync connector. Cleaning everything by blowing clean, compressed air (from a aerosol can) in every nook and cranny and trying to get air in between the mother board and the RF sheilding from all directions, blowing everywhere on the back case and on and around the buttons (that where still in the case) and on all connectors (both on the mother board and on the flex cables).
Then assembled everything again, making 100% sure that all the connectors where correctly seated. Turn it back on and loo and behold! Everything works like a charm!! I can turn it on and off, I can enter DFU mode and it doesn't reboot as soon as I unplug the sync cable!! Dang, I'm a happy camper now!! :D
PS. Just in case this haden't worked, my next attemt would have been to spray/use some 'electronic cleaning solvent'.