Shower Pump Surging Problem

[bathroomboy]

This is a shower pump I installed many years ago for my sister, it has been problematic intermittently, it is a Watermill 3 bar unit running 2 showers, it fed from a cylinder with surrey flange, I have tried an automatic air vent in the past and recently put in a reliance check valve but to no avail. Pump will run for about 30 seconds and then shut off and restart. To replace the pump with similar quality is quite expensive, any ideas?

 

[rpm]

Check filters are clear on inlets to the pump.