1991 Honda Accord. Intermitent speedometer

Were you ever able to pin-point the problem? My son's 2001 Accord is having the same issue.

if S button flashes and at the same time the speedometer is not functionning correctly, then the speedometer sensor is malfunction. The sensor is located at the transmission body above driveshaft.

I am no Honda expert, but a similar problem happened to my ex sister in-laws Ford Taurus. The speed odometer would jump around and it had trouble shifting (automatic transmission). After driving it I changed out the speed odometer sensor on the transmission and it's been fine ever since. I changed out the sensor because if the cars computer cannot tell how fast the car is traveling then fail safes in place go into effect and the computer will do it's best to keep the cars rpm's down based on the speed indicator and not let it shift or shift properly either. If it's getting false speeds then it will shut down the motor when the speed gets to high and allow it to run as the speeds fall (jumping speed odometer needle). It effects the transmission shift points also. It will either shift erratically or not shift out of say 2nd gear, so that the cars engine can't get damaged based on what signal the speed sensor is sending the computer.

Most of the time this problem is fixed by replacing the transmission control module (tcm). Sometimes you need a vehicle speed sensor too, but that is not as likely.

Locate a VSS (Vehicle Speed Sensor) from a local parts shop or junkyard and replace it, 90% of the time, that is the problem.

Speedometer cable needs replaced. It is very easy to do if you have a little common sense and a few common tools.
The cable runs from the speedometer to the transmission, Just purchase a new speedo cable and remove the instrument cluster, Look at the backside of the cluster to find the cable to the speedometer, Remove it as you follow the cable to trans replace the same way it was removed and reinstall cluster.

is your car not starting or speedometer does not work?
you either have a bad speed sensor or crank sensor

speed sensor maybe

check the sensors-most likely the sensor on the speedometer sending out false info

A green flashing "S" is similar to a check engine light. It is telling you that something in or on your transmission is not functioning correctly, such as the speed sensor or shift control solenoids. If it starts flashing and the speedometer stops working or the transmission shifts weird, take it to a mechanic.