If its the model I think it is then a bit ambiguous re venting so perhaps its detecting "air" on occasions?
Startup
The pump must not be started until the system has
been filled with liquid and vented. Furthermore, the
required minimum inlet pressure must be available at
the pump inlet. The system cannot be vented through
the pump.
The pump is self-venting and does not require venting
before startup.
In speed III, the pump is set to run on the maximum curve under all operating conditions. See fig. 14.
Quick venting of the pump can be obtained by setting the pump to speed III for a short period.
Startup
The pump must not be started until the system has
been filled with liquid and vented. Furthermore, the
required minimum inlet pressure must be available at
the pump inlet. The system cannot be vented through
the pump.
The pump is self-venting and does not require venting
before startup.
In speed III, the pump is set to run on the maximum curve under all operating conditions. See fig. 14.
Quick venting of the pump can be obtained by setting the pump to speed III for a short period.