Frequent "Invalid URL" problem on my MacBook

I have a strange problem accessing websites with my MacBook. I'll go to a website like Google, Amazon, Netflix, etc, and intermittently, I'll get the following error message:

Invalid URL

The requested URL "http://www.netflix.com/", is invalid.
Reference #9.3f5a9140.1291780004.110aa136 (this changes)

The error message appears on a simple white page, in black serif type. The next day I'll try again, and the same URL will work fine. The URL is always correct.

I have a 2008 aluminum unibody Macbook OS 10.5 Leopard.
I've deleted cookies, cleared cache, and updated all software, including Safari.
But the issue still periodically occurs.

Any insight would be much appreciated.

Thanks!
Rick