The easy: It's NOT the PSU. The either work or they don't. They don't freeze the computer.
The more difficult: Is it the RAM? Best answer you'll get is maybe.
You're correct… the MDD is supposed to have 500Mb SIMMS.
Under OSX. you can run 2x1Gb. However…
OS9 can only access 1.5Gb of RAM. You didn't say what else is in there but I'll guess it's 2x500Mb. So you have 2Gb in 3 SIMMS. It's possible repeat, possible that under OS9 you're running with only 1Gb of accessable RAM.
That's because the "last" 500Mb normally only gets partially accessed by the System in a very arcane way that has been discussed at length here in the past as some have tried (essentially in vain) to find a way to use the full 2Gb under OS9. So, depending on which slot the sticks are in and the alignment of certain blue dwarf stars in a galaxy I forget the name of, the machine may need to have all 500Mb sticks.
The proper recipe for the MDD is: 4 - PC-2700 2.5v, unbuffered, 8-byte, nonparity 184-pin DDR SDRAM
You can buy 500Mb sticks NEW for $15-ish. If you're only using OS9 (again, 1.5Gb max) you probably only need one.
OS9 is really memory-efficient and PT off-loads a lot of stuff onto it's own cards so it's very possible that you're only running out of memory as you say, every few days / now and then.
…or NOT. BUT, for $15 and ten seconds to insert it…
It's worth a try.