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Typically, the tachometer works off of the distributor. It reads the number of times your roter spins per minute. It has to have a power supply also, and a ground. I would begin checking the ground first because that it's usually is the problem.
If the vehicle speed sensor was the cause you would have a check engine light and the transmission would have sifting issues into overdrive, I would think you just have a failing tachometer stepper motor (tiny DC motor) that drives the needle..
Possible fault according to your input could be dry points or fault connectivity within the cluster. Try to look for dry solder joints on the sensors which give the input, or clean the connectors using dirt cleaners. As the condition improved after reassembly it cannot be a failure but a fault joint in the electronics. I request you take your time to redo the earlier work with more patience, use a lens to detect any loose connection, if possible use a multimeter to test the continutiy.Good Luck.
Do you have engine code showing by chance? It's possible the crank sensor is bad or the connection to the ecm or pcm to the instrument cluster circuit board needs a look. If you need schematics check out www.promanuals.net they specialize in Dodge Ram service manuals.
Try replacing the speedometer sensor, located on the transmission under the car. There will be a few wires going to it with a plug. The speed sensor is a retivly low cost part, and most of the time when it begins to quit it will give irratic readings on the dash.
Hope this helps!