I'm really embarrassed for not having thought of this months ago, but I suspect that you're going about this project butt backwards by assuming that you should NOT have a problem and trying to scope out why you do rather than trying to find out if you maybe SHOULD have a problem, so to that end I suggest that on your next run you do NOTHING after installing 10.6.3 but select your network during setup and go directly to a pure vanilla 10.6.8 using the
incremental Combo updater. (There's no body of evidence that suggests that updating incrementally is better than running the Combo, particularly in your circumstances, and, in fact, each incremental installation introduces new opportunities for errors, while running the Combo is a frequently recommended troubleshooting step following a troublesome incremental update.)
If your problem doesn't recur in the vanilla 10.6.8 you should probably begin by running Software Update incrementally followed by incremental migration to see at what point it breaks, but you may very well find out that 10.6.8 has, in fact, optimized your iMac (which was on the verge of vintage status when it was released) to death...that the only way around your problem is a new Mac.
Good luck!
It's a shame that so much rides on 10.6.8's shoulders.
