QPSK transmission and reception in GNURadio

I have implemented this:

Constellation plot of receiver side is same as that of transmitter but it disappears for some moment and is not continuous, It flickers and keeps on moving around and for a very short moment show a garbage plot also.

How to resolve that to get a correct continuous plot?

Never use a "Throttle" block in a hardware flow graph.

What it does is just slow down the processing speed "about roughly" to the requested rate on average. That will sooner or later lead to your computer not fetching the samples from your hardware source quick enough, and you run into overflows.

That is probably the moment when you see "garbage" plots.

Generally, 20 MS/s is a lot, and usually, hard drives aren't reliably fast enough at writing away that data. Also disable the file sink if the problem persists.

it disappears for some moment and is not continuous, It flickers and keeps on moving around

Congratulations! You discovered the wireless channel. That's what happens. Consult your wireless communications basics knowledge for more on that; this is not a programming problem :)