Two things come to mind...
1. I have heard some discussion on other forum about the water pick up problems on the coolingmist tank. Here is the
link. But one of the things to check out.
2. I believe you are the one who fitted a pressure switch inline with the IDC sensing wire. This may cause the some problem for the system to ramp up fast enough from 0% DC to 50% DC. This delay may be your problem as the system was seeing nothing and sudden expect to calculate the correct flow for a 50% DC.
Please disconnect the inline presure switch and see if the problem persists.