For the issue most describe, I don't think it is related to network congestion. One second I'll run a speed test and get 1.5 mbps down. Then data will suddenly stop working and I'll try to run a speed test, but it'll just try endlessly to conect to a server. After toggling airplane mode, resetting the phone, resetting network settings, or any combination of the previous and it finally gets working again, it'll pull down 1.5 mbps again. That's right after it wouldn't connect at all or pull down a single byte.
I've been so frustrated with this issue. I've called both Apple and AT&T tech support many times over the past 3 months, gotten a new sim card, got the phone replaced, restored, etc. and nothing seems to help. One second I'm pulling down 1.5 mbps, and then the next second can't even connect to a server, all with 4-5 bars of service and 3G icon.
I'm hoping that with the way this thread is growing more attention will be brought to the issue.
PS -- none of the suggested fixes works for me -- location services, turning off bluetooth, etc. had no lasting impact.