From the Nokia Support page:
Please send your phone to a Nokia Care Point for inspection if it has the following symptoms when using 3G/HSPA (High Speed Packet Access) data connectivity:
- Phone screen turns black but the physical keys under the screen remain lit
- The only way to recover from the condition is to remove the battery
A quick workaround to mitigate the symptom would be to disable High speed packet access via Settings > Connectivity> Admin. settings > Mobile data > High speed packet access > Disabled.
Note: With this workaround, you are still using 3G data connectivity, but HSPA is disabled.
And more detail comes from Nokia Discussions:
The lockups/freezes when using weak 3G/HSPA connections is a hardware bug. As I said, I took my phone to a Nokia authorized service center in Ukraine yesterday. They asked for a timeout, since it was obviously the first time they saw this device at all, and I had little hope since there is virtually no 3G in Ukraine. But today they called me to come in and I had a chance to speak to the technicians. They told me they found this very bug (freezes when using 3G/HSPA with the 808) in their internal support database and that there are instructions from Nokia to replace a capacitor on the main board!
This hopefully ends the saga of chasing this rather elusive connectivity bug down - if you're affected then make an appointment, under warranty, at a Nokia Care Point near you.
Comments welcome if you go ahead and request the fix. How well did the Nokia Care Point staff do?
PS. Gratuitous shot of the 808 motherboard, from here: