Whew, was waiting for someone else besides me to report this. My forum (www.enigmacomputerforum.co.cc) kept getting the E500s all day yesterday, then it was solid E500 until about 2 hours ago. Couldn't access the HH forums, couldn't get into the cPanel, nothing. My error log read things like "Could not allocate memory" and such, which confirms djbob's statement.
I use a php based forum client called MyBB, and I'm wondering if GZip compression is causing too much load on Stevie? If it is, (I'm not sure) I'll turn it off to help minimize these errors.