Converter electronics remotely mounted from the flow tube manifest noisy, oscillating readings when the cable used to connect the flow tube to the electronics does not meet the manufacturer's shielding and grounding requirements.

Poor grounding (earthing) causes noise issues which can affect the reading to a large degree, as much as you describe.

Pulsing flow tends to produce lower readings than normal but only at full flows.

Aeration (air bubbles) can cause unstable readings in mag meters, but generally the flow values are higher than reality, because the air bubbles are misinterpreted as actual liquid volume.

Coated electrodes in the flow tube can cause a lack of reading.

There could be a rag stuck in the flow stream/pipe, not necessarily in the flow tube, causing low flow and osciallation.

On one brand of magmeters, the loss of connection to one of the two electrodes would halve the proper reading.

Really noisy highly conductive slurries might need an AC mag, not pulsed DC technology.