Thanks, tacit. Your suggestion was one of the first things I checked; I have Earthlink as ISP and use its DNS servers, but also keep Google's and OpenDNS's server IP#s in my DNS list. As I'm still having this problem, a DNS issue seems not applicable.
As I mentioned above, the problem occurs on several Macs, but most frequently on an iMac G5 running Tiger; I haven't been able to get to Google on it for hours now. Before that Google access was intermittent, with problems frequently (but not always) reappearing when I tried to access search results. Meanwhile, an MBP running Leopard is having the same problems, but much less frequently.
I suspect some sort of issue with the wireless and ethernet configs of the Macs involved. I just cycled Airport off and back on again on the recalcitrant iMac (w/ both ethernet and wireless active), and now it can reach Google again (the MBP is usually wireless only). Let's see how things develop.
