My mid-2010 13" MacBook Pro with 2 2gb RAM chips occasionally freezes after putting it to sleep and trying to wake it up again. And after powering it off and turning back on again, the amount of RAM available changes. For example it will say it has 4GB of RAM, it will freeze up, I'll restart it and it will say I now only have 2GB. It says one of the slots is empty. It happens the reverse way as well. This happens once or twice a day. I have opened up the mac and tried starting it with each chip alone by itself in each slot and sometimes it works but other times it doesn't. Any ideas of what this could be? Is it likely something with the logic board or withe the chip itself?