My MacBook Pro (2007) on Lion has a strange behaviour.
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Sometimes Chrome browser stops working. When I then launch Safari, it can not load any web page.
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When I try to download a large file, it can be unresponsive.
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Sometimes working with PDF files (even small files) has the same result
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Sometimes I can not view files in my folders because Finder loads them very slow (it's showing spinner indicator).
I haven't found another way to resolve these situations besides restarting the laptop every time they happen.
What can be the issue? At first I thought about my HDD but my S.M.A.R.T. Status is Verified. Any additional ideas are highly appreciated!
Update : Activity Monitor shows 
Best Answer
Try to retrace your steps. What have you installed on you Mac since the freezes started? Some suspicious software/freeware from the internet, maybe some illegal software... If you can think of something like this, try to uninstall/delete the software.
Try to detect which process is running (Activity monitor) when the freezes happened. If you detect the mall function service or software, try to figure out if the software is installed correctly.
Worst case scenario, you can back up your data (not software!!) via TimeMachine or just an external hard drive, and completely reïnstall you OS. It quite radical (and unnecessary if you ask me if you can figure out the bad software/process described in the above paragraphs). In this way, you have a completely new OS and you can restore your documents and reïnstall your software (be aware on what you install).
My advice, figure out your bad piece of software/process and fix it. So you can keep your documents without reïnstalling the whole Mac...