Now that I'm consistently getting the 999 error using Yahoo search, it's time to try another search engine. Ask will be my default search engine for at least the forseeable future. I'll let you know how that goes. If it fails me miserably, I'll try something else.
BTW, the Yahoo error applies to both IE7 and Firefox, so it's apparently not a FF extension problem. If they think my IP is scraping them, then they are woefully mistaken, and should take another look at their coding. Nearly a year ago, I made the switch from Google to Yahoo as my default search engine, and I've been happy with that decision ever since. It's a shame that I can no longer use it, as I don't think Yahoo has all that many evangelists out there. Oh well, for the moment, at least, it will be Ask's gain.