After Backblaze install, Mac frequently hangs with spinning beachball

After installing Backblaze remote backup software and starting my first backup, I was away from a few minutes and my screen went to sleep. When I typed my password to log in, I typed Return and the system promptly hung with a spinning beach ball. I ended up doing a hard shutdown using the power key.

After restarting the machine and resuming the backup, I used the machine for a while (60 to 90 minutes?). This time I was typing in an app when the machine stalled with a hung with a spinning beachball and I ended up doing another hard shut down.

I immediately ran the Backblaze uninstaller, emptied the trash, and restarted the machine.

But the problem persists. After using the machine for awhile (60 to 90 minutes?), whatever app I'm in stops responding to the keyboard or mouse clicks. I can still move the mouse. If I move it over anything Finder, I also get the spinning beachball. Other apps still show the pointer when moused over. Sometimes the Dock works for another minute or two for switching apps and Spaces, then it stops responding too. Control-Option-Escape fails to bring up the Force Quit menu. I can't pull down the Apple menu to shut down the machine, because hovering over the icon just gets me a spinning beach ball and no response to clicks.

I checked the Console log, and found thousands of these messages, especially right before the machine hangs:

2016.07.24 9:28:33 AM com.apple.Finder[175] SystemFlippers: didn't consume all data for magn ID 0 (pBase = 0x100709198, p = 0x10070919c, pEnd = 0x1007091a0)

2016.07.24 9:28:33 AM com.apple.Finder[175] SystemFlippers: didn't consume all data for magn ID 0 (pBase = 0x1007091e8, p = 0x1007091ec, pEnd = 0x1007091f0)

2016.07.24 9:28:33 AM com.apple.Finder[175] SystemFlippers: didn't consume all data for magn ID 0 (pBase = 0x100709238, p = 0x10070923c, pEnd = 0x100709240)

2016.07.24 9:28:33 AM com.apple.Finder[175] SystemFlippers: didn't consume all data for magn ID 0 (pBase = 0x100709288, p = 0x10070928c, pEnd = 0x100709290)

2016.07.24 9:28:33 AM com.apple.Finder[175] SystemFlippers: didn't consume all data for magn ID 0 (pBase = 0x1007092d8, p = 0x1007092dc, pEnd = 0x1007092e0)

I checked the Backblaze website and didn't find this issue.

I contacted Backblaze technical support, which said they will get back to me on a weekday.

Luckily, I made a Time Machine backup immediately before installing the Backblaze software.

I used Time Machine to revert the file ~/Library/Prefers/com.apple.finder.plist. That didn't help.

I wondered if the problem might be in my Library/Caches/com.Apple.Finder folder, but Time Machine will not let me revert it. I have seldom needed to resort to Time Machine, and am not very good at using it.

Any idea how to troubleshoot and fix this problem?

Machine: Mac Pro (Mid 20101), 2.8 GHz Quad-Core Intel Xenon, 12 GB memory

OS: Snow Leopard (10.6.8)

Storage: 1 TB boot disk with 657 GB free, plus several other drives