Arduino Overload

Hello, I have an arduino Mega with the 4-platform expander shield and then two GPIO expanders on that (All sold here). I am not using anywhere near all of the I/O, either digital or analog pins that I have available to me. But after fooling around with it for a long time I realized that if I unplug any two pins on the board then it works. However if I have all my gravity sensors, relays, and LED's plugged in, when I plug it in the my PC the lights come on but then die after like 10 seconds and my PC does not ever recognize it. If I unplug any two peripheral devices and plug it into the PC, it works perfectly.

What is happening, I was under the impression that if you had that many pins available you could use them without problem.

HELP! because it seems like a problem with the micro controller but I am not sure. since it doesn't matter which pins I remove to get it to work.