Pump pulsing

I have a customer with a set of stuart turner 3bar monsoon pumps. One of the pumps is fairly new and problem free. the other however has developed a problem. ! The pump itself is via an essex flange flows down to pump, and cold via cold storage to pump, not a problem with pipework but detailing as much as poss !! The pump feeds 2 showers and works fine ondemand, full pressure and off when not inuse, but since last week has started to pulse when any other (hot) tap is used in the house, even though the tap is not part of the pump circuit !!
For example kitchen tap switched on, then off, immideately when tap is turned off the pump clicks on then off.
Now im not sure if for some reason it is detecting some flow movement in the cylinder, which to me indicateds maybe a faulty flow switch ???? but i have never fitted one of these pumps or have had to deal with one before. Has anybody had a similar problem, if so, what , where , how, and what was done ?? is the flow switch a replaceable part ?

I've got a Stuart Turner pump at home. We get the occasional airlock, and this seems to cause the pump to pulse whenever another tap (usually hot) is used elsewhere in the house. Problem persists until the last bit of air is out then all is fine again.

If the pump is positive head then as said it could be a bit of air or maybe the flow switch is dirty.
If it's negative head then the non-return valve on the hot water side could be letting by.

hi its a positive head pump. Pipework is all angled few degrees down to point of entry on pump, so surely air would find its way back to cylinder !! Stuart turner tech team said oh its the cylinder not refilling quick enough and movement of water in tank switching pump on, yet the other pump next to it is fine, he said to clean / check the flow switch it would need dismantelling ?? Not even sure what to tell the customer ?? well i would like to spend 2 hours stripping the pump then rebuild and it still doesnt work so paqy me or get a new pump or just leave it !!