Sony Ericsson W550i and bluetooth handsfree Logitech- connection problem

Hi
I have problem with W550i and Logitech mobile pro headset. First connection
works good, but after a period of time phone interrupt connection, then
connection again starts and so on. Sometimes the sign on phone for bluetooth
connection is on but there is no real connection. The main thing is that
when I need connection to have conversation handsfree I cant do it. It
takes me crasy, please help me.
I have tested headset with two others phones and everything goes OK, so the
problem is in W550i.
Any ideas, please!