the random flickering is mostly due to one of the two things...1) the speedometer needle became loose from some traumatic force since the needles themselves are sensitive, which is why when people change the gauge faces, they are warned not to disturb the needle or 2) your speedometer calibration is off which i suspect is the real reason.
there are two ways you can correct this problem. 1) you can bring the car to a shop and have them calibrate it which is pretty expensive or 2) find an undamaged instrument cluster and install it. however, if you do swap out the entire cluster, make sure you find one thats the same model as the one you have. the good thing is that if you are going to swap it out yourself, you can change the light bulbs behind the cluster to a different color setup.
the best piece of advice i can give is to bring it to a mechanic so they can tell you exactly what the problem is.