I would do as the previous poster suggested as well, but considering how little you seem to be running the system, I doubt that would be a problem.
Typically problems of this nature are often caused by a hard drive in the process of developing problems, a fault in the cabling to the hard drive, or an intermittent cracked trace (bad logic board).
There are only two tools that I know of that can address this properly. One is Apple's Hardware Test for system diagnositics run in a continuous mode to check the RAM and CPU, and the other is to use Scannerz to analyze the hard drive for developing problems with the drive and detect intermittent faults in the path between the drive and the logic board. Apple's Hardware Test should be on one of your OS disks and is available on the web, and you can obtain Scannerz from:
Home
With luck, you won't need Scannerz, and it will turn out to be something like a bad RAM chip or something trivial and easily fixable. With bad luck you'll get erratic readings during prolonged read/writes of the CPU/memory test which typically implies that a crack has developed on the logic board and it's making intermittent contact.
If it passes Apple's Hardware tests for memory and CPU, I'd move on to Scannerz. I would avoid generic tools like TechTools Pro or Drive Genius because they run generic tests on a hard drive, whereas Scannerz monitors hard drive timing and data rates and indicates oddities in time as irregularities. Unless a drive has blatant errors, TechTools Pro, Drive Genius, and even Apple's Hardware Test will miss these.
The good things about Scannerz are as follows:
a) If your in the US they offer tech support for their product and can assist you.
b) It can detect faults in the path from the logic board to the drive.
c) It includes FSE-Lite which can identify a process that's a disk hog (another source of spinning beach balls)
d) They have a free demo download of it available, but it's pretty thoroughly castrated unless your primary HD is 10G or less in size (unlikely for anyone these days) Maybe you'll luck out and find a problem within the 1st 10G though?
e) It supposedly (note that word: SUPPOSEDLY) has some support for SSDs. This is unconfirmed by me.
The bad things about Scannerz are as follows:
a) It costs money (but it's still a lot cheaper than Apple if you need them to troubleshoot something)
b) To get detailed tech support (advanced support?) you have to set up an appointment.
c) Unless you're fairly well hardware versed, some of the things they describe in their manual are likely over the head of non-technical people.
d) Probing a unit for cracked traces, bad cables, etc. usually requires opening the unit up.
e) It's manual is huge and the interface isn't terribly good looking (no razzle-dazzle, if that's REALLY important to you)
If you're lucky it will be something stupid like a bad RAM chip, an improperly seated RAM chip (worth checking), a failing hard drive (they're cheap these days), or a poorly seated cable to the HD (you did have it replaced once, right?) If you're unlucky it will be a failing logic board chip, a cracked trace somewhere in the logic board, or a system where some of Apple's overseas assemblers decided to save money by "improvising" their own cost saving measures by using inferior quality parts - Don't even get me started on that one!
Good Luck!
