Todays problems have been pinned down to PHP (imagine) causing issues within the Apache process and causing the whole site to die.
I've switched the way that PHP is called, and in theory it cannot now crash the entire server. Its vaguely possible that PHP itself still could die, but if it does you should be able to reload the necessary page (note, only Reader Comment system uses PHP at the moment) and it should stay working.
I shall keep my eye out, and apologies for the intermittent service today.
Steve