Server memory not enough?

That depends entirely on what sort of plugins you’re using and how much traffic you get 🙂

How much memory are you using now, or do you not know?

Looking at your other posts…

http://wordpress.org/support/topic/mysql-vs-ram-optimization?replies=1

You seem to be using under 50megs of memory, so that really shouldn’t cause things to crash.

What plugins are you using?

25 each, yes, and 25 * 6 is 125, so that could be the cause. You may just have too much going on at once.

Are your 6 sites getting a lot of traffic?