Solving the strange phenomena of the variable-frequency crude oil transfer pump

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I am using a multi-stage variable-frequency centrifugal pump with a rated flow rate of 72 m³/h for transporting crude oil. Due to unstable inflow to the connected separation buffer tank, variable-frequency control is generally employed, with the frequency being linked to the liquid level in the buffer tank; the maximum frequency is 50 Hz and the minimum is 30 Hz. Operation starts normally, that is, at lower frequencies until the rated liquid level is reached. Due to the minimum flow rate limit, we usually do not use 30 Hz but rather a value slightly above it, as the flow rate remains relatively stable at fixed frequencies. However, in recent times, it has been nearly impossible to determine a fixed frequency for the data flow; as the frequency fluctuates, manual control of the frequency is required. Please ask an expert to help analyze where the fault lies.