It might be but then again, everything should be the same across all the sites, right? Plus I would think the java errors would be logged in the error logs. makes it really hard to troubleshoot things without them andf I hate to enable debugging mode in wordpress because of the way it spits out errors on top of your live site. Still, we may have to resort to that to get to the bottom of this.
So the only time you have the memory error is on the 5th? and no time after, right? If so then its probably not the memory limit.
tim