Maybe 30% of the time, the system hangs during boot. There are 2 possible
points in the boot sequence where it occurs: Immediately before the login
screen appears, or immediately before my desktop is there. I'd love to provide
a dmesg log or something, but neither ctrl+alt+Fx nor SysRq keys work anymore
if it happens, I have to reset. (How do I get a log from the last boot ?)
Behavior of the machine and occasional stack traces I get displayed (but cannot
save) suggest to me the problem is graphics drivers related. System is Lubuntu
16.04. I can always boot to command line without problems, and it also never
happens afterwards, after it reached the desktop successfully.

lspci -v:

01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] RV280
[Radeon 9200] (rev 01) (prog-if 00 [VGA controller])
        Subsystem: PC Partner Limited / Sapphire Technology RV280 [Radeon 9200]
        Flags: bus master, 66MHz, medium devsel, latency 32, IRQ 16
        Memory at d8000000 (32-bit, prefetchable) [size=128M]
        I/O ports at c000 [size=256]
        Memory at e9000000 (32-bit, non-prefetchable) [size=64K]
        [virtual] Expansion ROM at e8000000 [disabled] [size=128K]
        Capabilities: [58] AGP version 3.0
        Capabilities: [50] Power Management version 2
        Kernel driver in use: radeon
        Kernel modules: radeonfb, radeon

01:00.1 Display controller: Advanced Micro Devices, Inc. [AMD/ATI] RV280
[Radeon 9200] (Secondary) (rev 01)
        Subsystem: PC Partner Limited / Sapphire Technology RV280 [Radeon 9200]
(Secondary)
        Flags: bus master, 66MHz, medium devsel, latency 32
        Memory at e0000000 (32-bit, prefetchable) [size=128M]
        Memory at e9010000 (32-bit, non-prefetchable) [size=64K]
        Capabilities: [50] Power Management version 2