Verify failed while starting debug, but Memory Test passed.

quartus version: 13.1.0 build 162

board: terasic DE0-Nano

hw: nios2 system with user components, and a lot of FPGA logic.

sw: uC/OS with C++ code.

when I start debugging, it says verify failed during downloading about 9 times out of 10 tries, that is, it can download and debug correctly few times.

Using cable "USB-Blaster ", device 1, instance 0x00
Resetting and pausing target processor: OK
Reading System ID at address 0x0F010000: verified
Initializing CPU cache (if present)
OK
Downloading 04000000 ( 0%)
Downloading 04010000 (44%)
Downloading 04020000 (89%)
Downloaded 144KB in 0.8s (180.0KB/s)
Verifying 04000000 ( 0%)
Verify failed between address 0x4000000 and 0x400FFFF
Leaving target processor paused

And, I have created a Memory Test Small software project from the template, runs in an on-chip ram, for testing the SDRAM, it passed the test many many times, seems there is no error about the SDRAM.

             <---->   Nios II Memory Test.   <---->
This software example tests the memory in your system to assure it
is working properly.  This test is destructive to the contents of
the memory it tests. Assure the memory being tested does not contain
the executable or data sections of this code or the exception address
of the system.
Press enter to continue or 'q' to quit.
Base address to start memory test: (i.e. 0x800000)
>
0x04000000
0x04000000
End Address:
>
0x05ffffff
0x05ffffff
Testing RAM from 0x4000000 to 0x5FFFFFF
 -Data bus test passed
 -Address bus test passed
 -Byte and half-word access test passed
 -Testing each bit in memory device. . .  passed
Memory at 0x4000000 Okay
Press enter to continue or 'q' to quit.

Why?