Not too familiar with how the A3000 manages memory, but I'm thinking that you've probably got a loose chip, or a failing chip somewhere in the bunch, which is causing the entire bank of memory to drop out.
I'd start by throwing every memory diagnostic utility you can find at the thing, and maybe contacting Mechware or another vendor, or looking on one of the 'bays for a matching set of chips. I'd start by swapping these chips for a different set and see if the problem reoccurs, that would narrow it down to a chip issue or a MB issue.
Another thought - can you try running with 8MB, and then with the other 8MB of chips? See if at either time it drops to 4MB? At least by doing it this way you could narrow it down.
Could it be power supply related? A weak or failing PSU not providing enough juice for all the chips?
Good luck!
