There is timeBetweenTransactionsMillis parameter in the tcp thing to control silent period between different requests to the same host. Modbus - Bindings | openHAB thing
Default is 60ms but perhaps it is too short time for the device to recover.
Would that work out?
EDIT: @rossko57 has also written great write-up of different parameters and considerations in Modbus performance management in openHAB 2