profibus and satelline

Hi.
I use satelline 3AS modems for wireless link between two simatic
stations S7300 with cp343-5 and s7400 with cp443-5.
They both are configured as master at 9600kbps.
like here http://republika.pl/asdfg3/pb.JPG
Usally this connection work correct, but sometimes
cp443-5 is still transmitting data and cp343-5
signaling BUSF. Then i must (at this same time) restart
both of modems.
I'll be grateful for some advice.
Thanks.