I have a MPS400 instead of the MPS 200 (only difference is the ranges go higher than 200LPH and don't read accurately at lower rates down to 50LPH like the 200 does)
I had originally connected the MPS directly off the port of a radiator with a rigid fitting and had similar issues as yours.
After re-reading the manual, i saw that it needs some length of relatively straight tube before and after the MPS to get good readings, as it is pressure differential based and is easily disrupted by having too much turbulence in the flow. Six inches of tubing added between it and the rad solved my problems.
As I wrote, I have appx 200mm straight horisontal petg tube before the sensor.