A high pressure centrifugal pump is under development for a new rocket engine. During the performance tests, low frequency oscillations of high amplitudes were observed in pump inlet and outlet pressure measurements. From the FFT analysis of these pressure measurements, the significant frequency content was observed to be ~10% of operating speed of the pump. The tests were repeated with changes in test conditions to identify probable source for the oscillations. Internal flow re-circulation in the pump diffuser channels was identified as the source for the low frequency oscillations. Based on further studies, diffuser geometry design was improved with reduced channel diameter to resolve the issue, in two phases. In the first phase, existing pump casing was salvaged by installation of plugs in the existing channels to reduce the flow passage diameter. Tests were repeated with the plugs inserted in diffuser channels which successfully eliminated the low frequency oscillations. But due to the introduction of the plugs there was reduction in pump head. In the second phase, a new pump casing was realized and tests were repeated. These tests re-confirmed the effectiveness of the modifications. The diffuser pressure recovery coefficient was observed to be an important parameter for the onset of oscillations in the pump. This paper presents the details related to the pump geometry, test results for both the initial and modified diffuser configurations.