"Sounds as if your liquid is contaminated with microscopically small bubbles!"

Also called turbulence. In simple terms a flow meter is very effective when handling "laminar" flow. Most flow meters ( particularly the turbine types )come with a installation guideline that specifies a minimum straight run of pipe devoid of any bends / elbows both upstream and downstream of the flowmeter ports. Hope your installation complies with this. Turbulence can also be caused due to aeration ( entrapped gas in fluid )of the fluid - in such an event the meter reads more than the actual.

Also be careful not to use reducers in the inlet of the flowmeters - the edges of the fitting can cause turbulence. Idea is to have a very smooth fluid conductor in the inlet side.

How stable is the viscosity of the fluid that is being metered? Flow meters do have a specification for this and you can check on that.

By and large most of the above problems are non-existant with the gear type or screw type flow meters which are positive displacement type.

As another option you can try to transmit the pulses in differential mode rather than single ended.

Raghu