Spinning Beach Ball in Safari, relentless, despite passing hardware diagnostics

Question

My early 2008 24" iMac gets the spinning beach ball and stalls out, I use Safari constantly so that's where I'm noticing it the most.   When I reboot the computer, it initially works, then after a few minutes it starts spinning again. Sometimes my pages load, sometimes not and then I have to close Safari and open it again to get it to work.
I installed Smart Utility And Smart Reporter, checked my harddrive with those programs, it passed all the tests.  I run Onyx weekly.  I check my activity monitor, I have plenty of harddrive space, my CPU usage is low, I don't have a lot of open programs, but I still get the beach ball.  When I access the Force Close Applications, it shows Safari not responding.  Sometimes my third party MailPlane will also show that it isn't responding,
I've read everything I can think of, can't figure it out.
I do have a replacement Seagate Barracuda 7200.14 (AF) 500 GB in this computer.  I replaced the original harddrive two years ago.  I also have 4GB of RAM, 2 GB in each slot.  I thought perhaps I was having harddrive failure again, but every test I run says my harddrive is okay.
Any help would be appreciated.

Answer 1

When you next have the problem, note the exact time: hour, minute, second.
If you have more than one user account, these instructions must be carried out as an administrator.
Launch the Console application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Console in the icon grid.
Make sure the title of the Console window is All Messages. If it isn't, select All Messages from the SYSTEM LOG QUERIES menu on the left. If you don't see that menu, select
View ▹ Show Log List
from the menu bar.
Scroll back in the log to the time you noted above. Select any messages timestamped from then until the end of the episode, or until they start to repeat. Copy them to the Clipboard (command-C). Paste into a reply to this message (command-V).
When posting a log extract, be selective. In most cases, a few dozen lines are more than enough.
Please do not indiscriminately dump thousands of lines from the log into this discussion.
Important: Some private information, such as your name, may appear in the log. Anonymize before posting.