Error 1634955892? Something else?

Here's a particularly thorny problem i've been having with my mac lately. The problem has manifested itself in many ways:

1. Problems began when i noticed that certain web sites were taking an incredibly long time to load. They seem to partially load, and then get stuck. When they get stuck, i reload them a number of times, and perhaps, after watching it churn away for minutes, they may finally load. At first, i thought that this problem was restricted to overseas US web sites only (as local NZ sites all seem to be loading as fast as ever), so I got in touch with my ISP to see if there were any issues. The reply: none. Two weeks later i'm still having this problem with many US-based web sites (apple.com, cnn.com, nytimes.com, etc.)

2. Then i began noticing other problems: Software update: I recently used Software update and was told i needed to update some software, which i tried. Software Update downloaded the software but it wouldn't install. Instead i got the following dialog box: "The update, (x), cannot be installed. The update could not be verified. It may have been corrupted during downloading. The update will be downloaded and checked again the next time that Software Update runs". and "An error occurred during installation." After a few attempts this week, I went to the Apple support page today and downloaded the new iTunes update. Software Update again returned the same dialog box as above. So i downloaded the update from Apple's support page to my desktop to manually update iTunes. As normal, it downloaded as a dmg file. When double clicking on the dmg file, i get a dialog box that says "Warning: the following disk images couldn't be opened: iTunes 9.1.dmg not recognized."

3. This morning, Software Update didn't even register that there were new updates available (e.g. OSX 10.6.3, iPhoto).

4. A few days back I was in the iTunes application (v9.0.3) and tried accessing the iTunes Store. Instead of getting to the store i get the error message "The iTunes Store can't be accessed." So i'm unable to even get to the Store, though every thing else in iTunes is working fine (e.g. iPod sync, listening to music, etc.).

5. I've tried to update several different applications by using the "Check for updates" command from within the application. I first tried this with iTunes and got the dialog box that said "Check your internet connection", which i did, and all seems fine. I've tried using the "check for update" feature with other applications as well (e.g. Toast, Bookmarks) and all return similar messages like "the update cannot be accessed. Check your internet connection."

6. Toast's error message was a bit different. It said "Update check failed. Please check your internet connection then try again. Error code 1634955892", so at least i had a clue.

7. I searched the apple forums, and googled the error code but basically couldn't find a fix. There's some older info (e.g Roxio's site, iDVD apple forum, etc.), but the problem seems to be an OSX problem related to internet connection and isn't linked with any one application. I'm still experiencing the slow loading (or non-loading!) web sites in Safari (the problem with slow web site loading also happens in Firefox). One fix suggested that the problem could be fixed by entering "cd /System/Library/PrivateFrameworks/
sudo mv HIServices.framework HIServices-old.framework" into Terminal. No go there.

8. I've done all the normal things like 1. repair permissions, 2) deleted caches, browser histories, etc using Onyx, deleted a number of suspect plist files, etc. Still the problem persists

Does anyone know why this is happening, or better yet, how to fix it??

many thanks,

todd