Ive got one RB433AH and put it @ 800Mhz. Very unstable, but it works fine @ 680Mhz. Power supply is 18v 3.5A. Didnt have another one to test, but is there some sort of throubleshooting to apply here? Please.
edit
Aye, forgot to mention that was using ROS 3.10 also…
Thanks
Ozelo
Guess Im the only one who got a RB433AH very unstable running at 800Mhz… 
Greetings!
Excuse me if I start on the basic side, but 800MHz is overclocked. Gonna create some additional heat on the CPU. Maybe a fan across the CPU will help?
I mean unstable as it cant even boot properly. Heat sink is cold after all.
Try to boot it without any mini-pci devices plugged in.
also no microSD card … some are incompatible
Didnt even put those things in. Just got the board from the box, plug the console and then turn on power supply. Via console ive setting the 800 MHz and then let it run. Didnt see any boot up ok. Puting back to lower clocks (everyone than the 800) and it works fine. Console show many memory dumps and kernel panics while @ 800MHz. Otherwise it runs smoothly on any other settings except 800MHz… This board have a small heat sink and didnt came with fans at all. 
edit Detail: Some times after a crash it roll back the settings to a lower clock it self.
Hello, I have 2 x 433AH as well. They don´t even boot properly with 800 Mhz. All I get is a system dump at startup. And belive me, the CPU is cold. I set them to 600 Mhz and thery work fine, but I have one 411AH and this one hangs up every time when I want to scan with the Wireless-Cards. I will change them this week. Hope it works then. The performance is OK when it works…
800MHz is overclocked, and the CPU obviously won’t handle it. 680MHz is not overclocked, and it will work.
I have not used this, but I know the procedure. The CPU speed rollback is a lockup precaution. Once you set the speed, you must reboot the unit. If it comes up at the new speed, you must reset the speed again. If it doesn’t come up, it is supposed to rollback to the old speed.
EDIT: Don’t want to step on your post, heiko_s, but I agree. If it won’t, why say it will?
i’ve the same problem.
Booting on 800MHz isn’t possible.
Okay - CPU is overclocked at 800MHz but WHY tells MT on the routerboard website that it WILL work with 800MHz - i thinke there was no testing at all for this feature.
i hope they’ll wipe this option out from the routerboot that noone accidently uses it.
I think the RB4xx series was very hot cooked - even the 411 have problems with powersupplie - with rb133 and rb532 we never ever had problems…
maybe will be fix by adding a 700Mhz option…