We present numerical and experimental study on the origin of fluctuations in the idler power in partially degenerate intermodal four wave mixing in a 1 km long few mode graded index fiber where both the fundamental and higher order modes are excited in both pump and signal wavelengths. Numerical simulation is carried out in the presence of different types of mode coupling processes and group delay fluctuations in order to independently predict the influence of these phenomena by considering the appropriate statistics of the corresponding processes. We propose a quasi-distributed model by choosing the locations of specific coupling events statistically, in correspondence to their correlation lengths. Idler conversion efficiency is experimentally evaluated for different detuning frequencies between the signal and pump through multiple trials, and it is found that even though distinct features of maxima and minima are retained, there are fluctuations in frequencies corresponding to maxima and minima. These experimental results are compared with those from numerical simulations in order to identify the origin of fluctuations. © 2020