If two monitors from the same connection appeared to fail I would then think it is the video card itself.
With respect to mini-displayports, I'll mention the following, but I am not sure this applies to you since you were getting signals to the monitors. Make sure the mini-displayport plug is pushed firmly in so that essentially all the metal part disappears inside the socket. Mini-displayport plugs can be a little tricky in that you need to make sure the plug is fully seated in their socket.
You could try another cable or adapter as a cheap experiment but I also doubt this is the problem here too.