Error when using network (tiny6410)

I have faced strange problem.

case 1) ping 6410 to PC
result : failed

case 2) PC to 6410 -> 6410 to PC
result : success 

case 3) PC1 to 6410 (continuously)
        6410 to PC2
result : before 6410 send ping, PC1`s ping works well
         as soon as send ping, PC1 says 'Unreachable'


I uploaded test file about ping log.


Not only on ping work, also doesn`t work on other network like socket or
wget.

If 6410 send socket first, it stops. If pc send first, works well.

I have setup apache so if using "wget http://(PC IP)" download index.html.
But it only works after PC send ping to 6410.

I have searched forum but I cannot find similar issue.
If anyone knows about this, give me a answer.

Thanks.

I checked using Wireshark.

When ping works well, I checked it on wireshark.
When 6410 downed, wireshark showed no data transfered.

Change other router and tested but still has same problem.