Recurrent beachball in all apps in Lion, eventually freezes iMac

Pardon the details: I want to be concise but also show that I have combed through many posts here and tried many of the remedies suggested. I also wanted to be as detailed as I thought I needed to be to communicate the problem accurately.

The problem:

I have two accounts, my everyday one and a nearly unused admin one I set up after installing Lion. Whether I start them in safe mode or not, they eventually beachball. The beachball will only show within the app, but although the arrow will appear outside the app window, it can't click on anything and eventually freezes or I just give up and reboot.(Anecdotally, the beachball seems to appear later in safe mode and/or the admin account -- I'm not sure how significant that is.)

I even left the machine at the login screen overnight, and when I clicked to login this morning . . . beachball!

Note: my Boot Camp partition runs fine; my Safari-only guest account also doesn't seem to crash (I'm typing from it now).

The context:

I made many software changes over the past few days; I had an Evernote file full of minor tweaks and changes I had run across, adding and sometimes deleting new apps -- nothing shady or major (for example, I started backing up with Crashplan . . . but it had not finished). Obviously, though, I may have screwed something up there, though in Safe Mode, most of those changes should be excluded from the runtime environment. Previous to this, I've had an ancient problem with one gigantic/complex spreasheet beachballing OpenOffice and LibreOffice, but I was always able to force quit and use other apps.

Remedies/Sugestions I have pursued:

I have no issues with HD space or memory (16G mem, several hundred GB unused on HD) and can't detect any audible, abnormal HD noise.

I have searched through these forums and tried

• watching System Monitor and iStat for abnormal CPU or memory surges around crash points, and nothing suspcious happens, not even any page outs, other than an app goes into Not Responding status, then the beachball hits and all apps freeze (including Activity Monitor
• In Recovery Disk mode, Disk Utility found no errors on Verify Disk and only miscellaneous permissions to repair
• Detaching all peripherals (except my Bluetooth mouse and keyboard, as well as my ethernet connection)
• Apple's AHT software test, including the hour-plus extended test with peripherals detached: no errors
• Cleaning all caches with Onyx (to treat possible file corruption)
• Scanning the whole disk with Sophos for viruses and malware, nothing found
• Looked at Console logs around the time of beachball appearance. I am not very good at "Console" interpretation. I don't see any obvious crash precursors there, but I'm not really even sure what I'm looking for. I can post some partial logs of activity just before the appearance of the beachball if anyone thinks that would help.

I have seen four other recommendations that might apply to my situation:

• run unix shell commands, such as the ones found in this thread, and post the log
• uninstall all third party software (not looking forward to that!)
• clone system to backup HD and see if it works there (I have a HD large enough to do this, but Time Machine's on it and taking up too much space for this to work)