Mysql Cluster Backup Error (785)

[hoo.smth]

Hi,all
I can't backup my cluster for many days.
The error is as follows:

ndb_mgm> start backup
Waiting for completed, this may take several minutes
Backup failed
Node 3: Backup 48 started from 1 has been aborted. Error: 785*  3001: Could
not start backup
*        Schema object is busy with another schema transaction: Temporary
error: Temporary Resource error


It's not always failed, when I'm very lucky,i can backup successfully.
I have googled the whole day, but still haven't solve it.
Thanks for reading.

hoo.smth
2011-7-20