Do you have an adapter with 3 LEDs?
Which of the 3 LEDs turns off when the connection gets lost?
Do you have another master on the MPI - Bus?
If you have another master on the bus it could happen that both masters (e.g. a MP370 - Panel and the RS232 - USB adapter) get a conflict because there can be only one master on the bus.
If it only happens with huge VATs try to increase the percent level of the cycle time for communication.
You should be able to change this level in the harware configuration (see attached file)
I'm sorry. The screenshot is from a German version but I hope you can find the menu in an other version too.