My speedometer when running is going in and out

Well it could be the speed sensor. But I would think that either the check engine light would be on or at least the ABS light would be on. If they are not on, then I would suspect a problem with the instrument panel or the wiring to it. Are either of those lights on? If not, do they come on for a second or two when you turn the key on but does not start the engine?

I would start by looking at the harness that goes to the speed sensor on the transmission. That sensor is a simple pulse generator with a low signal, a corroded connector could cause problems. To check if it is the gauge or the sensor easily would be to connect a scan tool and watch the raw live data regarding vehicle speed, if the signal drops out when the gauge does it is likely the sensor/wiring. If the signal stays steady but the gauge varies it is likely a failing stepper motor.