I would start with checking to see if you have the required memory for the IOS version you are using by doing a show version. This will also tell you if your router is recognising the that is installed. It should match the amount that you ordered with your router.
You can also check to see if you IOS is corrupt by using the verify command.
Another thing to do is to boot the router with a terminal emulator connected to the console so you can watch for errors upon booting. If you have a problem with memory it may show here.
I could not find the test memory in my 2811 either so it must have been removed or not be supported on this model router. It is however in the Command lookup tool under 12.4 IOS so the command should exist.
Like Paolo, I too suspect an IOS in such cases, assuming the hardware isn't faulty. In your case, your 2821 is running a recent "T" version but there are a couple of newer versions, including a newer one for the version you're running, 12.4.22T1. You might upgrade, or downgrade to a version of 12.4, which might be a little more stable, if you don't need a feature in the "T" versions. Also, you could update your ROM software, which notes "DRAM access optimization" in the latest.
I agree if you suspect the IOS then upgrading it might solve your problem so you can try the update 12.4.24 that is out. It may be a bug with one of the features you are using which is hard to tell with the info posted. You may want to check the bug tracking system to see if your specific problem has been reported.
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