I’ve been having this issue for a while now. Noticed that it happend after I changed my alternator. The vss was changed and works fine intermediately. If your foot is on the excellerator it jumps up and down until you let go of the pedal. If you floor it it works normally as well. So it only works full throttle and when the gas pedal is released. Also when the speedometer is jumping around you can feel the engines rpm fluctuate with the speedometer jump. Not so much the rpm gauge. I have a new vss from 1 a Auto and I put the old one back in. Same results. I also have code p1574. I changed the sensor that has the gear attached that is driven off of the transmission diff gear. any idea what to check?

FINALLY FIXED! So after troubleshooting the car for a whole day. I exchanged my alternator. Found my battery was about done, (345CCA original battery was 700CCA) changed that as well. Still had the same issue but worse. So put my code reader on it again and seen I had a code P0462 and code P1574. Not once did my fuel gage read faulty. I would drive till 1/4 tank and fill up. The gage would reflect accordingly. So I changed the cluster with my other 2002 Altima S. Same results again. Speedo jumps while moving and at one point it didn’t move. So I figured ok time to change the FUEL PUMP ASSY. Changed that and couldn’t believe that fixed my speedometer issue. Make absolutely no sense! Now if my gas gage was going crazy I would of changed it a long time ago. Hope this helps someone who has the same issue. I almost gave up and was gonna bring her to the dealership.

Update to this post. Seems the speedo only worked correctly for 1 day. The next day it was back to acting up again and still had the same two codes (p1574 & p0462) Did some research and found the ground cables on the timing cover were for the tcm and ecu. Checked those out and they were loose. Tightened them up and cleared the codes. Been working good so far over 100 miles later.