There are many possible answers to the questions asked about the erroneous taxi data.
The GPS receivers may not be getting valid data due to being in an urban canyon (surrounded by tall buildings that create multipath, or blocks the view of the satellite constellation enough to prevent the receiver from resolving its position).
You could also have faulty receivers, or interface parameters if they are recording data to a PC, or tablet of some sort.
Speed, or velocities are a different matter. In the testing that we have done, we have found the velocity calculated by the GPS software was approximately two miles per hour slower than that of a calibrated speedometer.
Some of this can be attributed to the GPS measuring the distance between the points over a model of the earth, rather than speed over a given measured surface.
There is also the latency factor with GPS measurements, and calculations.
If your GPS update rate is 1 Hz. (One time per second) your velocity will only be calculated at that rate if a valid GPS position is collected for that time interval. In other words, if you have a position every second, you will get a calculated velocity between two consecutive points.
With latency, what you will see is the position collected will be behind you when you receive an update. The distance the position is behind will depend on how fast you are traveling, and how long it takes the computer to calculate where it is.
To simplify this, if you are traveling at 60 Miles Per Hour (88 feet per second), and your GPS unit updates once a second, and the latency of the system is 0.25 seconds, you will be 110 feet beyond the position displayed.
I have seen GPS velocities remain the same over several seconds, or even minutes when there were no valid GPS positions collected. This is more a function, or lack of function of the software being used.
I would first check to if the interface parameters are set correctly between the GPS units, then I would check to see if the GPS units are functioning properly in an area where there is a clear view of the sky in all directions.
You can also compare the number of satellites in view between the different units when they are in close proximity to one another to see if the GPS units are acquiring the same data, from the same number of satellites, and the positional quality (PDOP, HDOP, VDOP) are close to one another.