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Your other sites have likely increased their usage to a point where your total memory requirements are bigger then the memory allotted by your hosting company. Contact them about getting more ram added to your account.

http://stackoverflow.com/questions/12324622/system-outofmemoryexception-on-shared-hosting

	
The problem with it being a random thing is it's likely to happen again. It's likely that your other sites memory usage changes over time so you'll likely still get the same error in the future (and more often if traffic increases). It comes down to are you making money from any of these sites OR does it matter in some other way if someone sees this error? If not.. well just accept it will happen again in the future. But if it does matter then pay for better hosting either with your current provider or with a new one.