Reduce "flicker" in spectrum analysis / spectrogram
My suggestion is to low pass filter the FFT slices.

Suppose your current FFTs are:
$$
X(n,k) = {\tt FFT}\{x[n] \cdot w_n[n]\}
$$
where your signal is $x[n]$ and the window $w_n[n]$ enforces how much of the signal you want to include in the FFT at time $n$.

Then form:
$$
Y(n,k) = \alpha \cdot Y(n-1,k)  + (1-\alpha) \cdot X(n,k)
$$
where $\alpha$ is a fraction between 0 and 1 (probably around 0.7 to 0.99).

This will effectively apply a first order lowpass filter to your FFTs.



Another possibility:

$$
Y(n,k) = {\tt median}\{X(n,k)\}
$$

where the median is taken over the last $M$ values of $X(n,k)$ i.e. $\{ X(n,k), X(n-1,k), \ldots, X(N-M+1, k)\}$.