It sounds like you keep catching FN+\'/?\'.
The yellow \'Num\' above the \'/?\' key refers to a Numlock, somewhat similar to the PC keyboard, except on the Zaurus it acts like an FN lock. To activate it you press FN + \'/?\'. When activated there should be a little \'N\' to the write of the clock on the taskbar and all of the yellow characters will be used by default. To deactivate it press FN+\'/?\' again.
As for the memory shortage messages, Qtopia does seem to be over zealous when it comes to detecting a shortage of memory. I find that it even warns you when there is only a momentary spike in memory usage, but you could try reducing the number of applications that you have Fast Loaded. I think by default the Sharp ROM comes with Calendar, Addressbook, Todo and Email set to Fast Load. I only keep the Calendar and Addressbook applications Fast Loaded myself.
