Oh OK then you do know how to do the usual troubleshooting stuff. (I forgot to mention the 'other PC' thing.)
How about this. Unplug all shields/wires and put in just one led/resistor. The next time you manage to get it connected, immediately reprogram it with nothing but a Hello World Blinkie sketch. Then try a few reconnects. If that doesn't work -reliably- then it might indicate -either- a bad board, -or- some other more fundamental software incompatibility. Basically, keep stepping backward till you find something that works, then step forward till it fails again..