Listener restart hangs if a transaction to an heterogeneus service is opened

@as400

Hi,
my env is 11gR2  (11.2.0.3) on Windows Server 2008 64-bit.

My listener stop-start work fine, but in following conditions start hangs in undefinetely way:

1) open a transaction to an heterogeneus service, say

SQL> select count(*) from <library>.<table>@as400;

(where AS400 is an heterogeneus service correctly configured as follow

 (SID_DESC =
      (SID_NAME=AS400)
      (ORACLE_HOME = <myOH>)
      (PROGRAM = dg4odbc)
 )

)

2) stop - start the listener

lsnrctl stop
lsnrctl start

3) the listener start remains pending till

SQL> rollback;
SQL> alter session close database link as400; 

Can you help me ?
Thanks
Renato