PLC communicatio error

Hi
I'm using the 3964R protocol to communicate with a siemens PLC.
Sometimes after the PLC sends the host a telegram the communication between the host to the PLC breaks, the PLC continues to send NAKs.

example:
host:<02>

PLC: <10>

host:<00><00><41><44><08><03><00><02><ff><ff><00><00><01><00><10><03><1e>

PLC: <10><02>

host:<10>

PLC: <00><00><00><4c><10><03><5f>

host:<10>

PLC: <02>

host:<10>

PLC:<00><00><41><44><00><00><00><08><ff><ff><00><53><11><98><00><00><02><00><00><01><15><00><00><00><20><10><10><10><03><f2>

host:<10><02>

PLC: <10>

host:<00><00><00><00><10><03><13>

PLC: <10>

host:<02>

PLC: <15><35>

host:<02>

PLC: <10>

host:<02><00><00><41><44><08><03><00><08><ff><ff><00><04><12><00><02><00><08><00><01><00><00><00><00><00><00><00><10><03><08>

PLC: <15><34>...

Does anybody have any idea what is the problem and how to solve it?
Is the byte after the NAK an error code?

Thanx