My engine light is on, my speedometer sometimes works and sometimes doesn't register, some times it bounces back and forth.

link with some info about faulty speedometer.
https://wheelzine.com/speedometer-not-working-properly

https://maxima.org/forums/4th-generation-maxima-1995-1999/311797-fixing-speedometer-odometer-work-sporadically.html

This may not be of any help but I had a 94 and the speedometer kept messing up. One day my wife was in it and she was stopped at a red light and it was bouncing around 30 MPH. She took it to the dealer we bought it from. They told me it is run off electrical signal and the alternator was bad. They changed the alternator and we never had the problem again. Just a thought check your alternator see if voltage is good are bouncing around. You can check it at the battery while it is running. I have never heard of anyone else where the alternator effected the speedometer but it is what I was told and they did change the alternator.
I will say I do not remember the MPH being 0 while running.

see if you can rock the tranmission back and forth pulling on cable what happens is they get a bur on the gear that wont let itout

Disconnect the negative cable for a few about 15 minutes. The Reconnect it and let the ECU(Engine Control Unit) reprogram itself. Sometimes this works. But sometimes it does not and you will have to see a professional to check the ECU to see if there are any electronic problems. Try to take it to a good mechanic and see what they are doing. Just watch ...if you know anything about cars or you don't will not matter They will get nervous anyway and not try to cheat you. Just ask them to notify you when they will work on your car.

May be vehicle speed sensor. Any codes?

Take off the belt & feel for the water pump bearing

check your alternator..take it to autozone or advance and have them test it while the car is running its free or charge your alternator may be power surging and cause the guages to go crazy like mine did and was the alternator give that a try

It would actually be the cable if cable driven

have the fault codes read
the speedo gets its signal from the VSS ( vehicle speed sensor) and that sensor also reports to the transmission control module
So check for a problem in that area

Possible ground wire fault with dash board unit. If it is a cable operated spedo, the cable end (small square cap) may be rounding off on one end.