This is driving me nuts as XBMC keeps crashing for me as well. I have come to the conclusion that this is due to the heat, as it has been crashing regularly in the very hot weather we have been having in the UK recently.
I am also using raspmc R4 and have just noticed that it has been over clocked to 800 MHz. I have just removed that to see if it makes any difference, you could try that and well.
I feel that you and me are one of the unlucky ones that have a pi that needs to run at a reduced speed of 700MHz. I found that my pi is extremely sensitive to the power supply and I thought had found the answer by powering the pi from the USB port on the TV (try powering the PI from a USB port on a laptop or PC also works). But during the hot weather that no longer works and it keeps crashing again. It does not work with my Nexus 7 2A power supply either.
I have found this thread http://www.raspberrypi.org/phpBB3/viewt ... 29&t=14489
"Fixing 1.8V power rail design error" which is a very interesting read and supports my over heating theory.
I suppose I could consider sticking various heat sinks on the pi.
I am also considering sending the pi back for warranty repair and hope they send me a new one that is not prone to crashing.
I have not measured the voltage on my pi yet.