USB ports if have too much current drawn just shut down. If your laptop
is throwing a hissy fit from the RAMPs try testing something like an external
DVD burner which draws a lot of juice.
I assume you've tested different ports on the laptop? I'm more inclined
to think the shutdown is because of a short somewhere between the RAMPs
and the laptop. Shutting down like that is the sort of measure a good PSU would
do to prevent a damage.
You can actually use a portable usb battery pack to power the RAMPs and Mega for
testing and work. Plus if the battery pack, thankfully cheap now, blows up you'll know
the RAMPs or the Mega are duds.
Andrew