Speedo doesn't work all of the time.

1995 altima 135k, speedo doesn't work all of the time. It is either fine, or sits at zero. When speedo doesn't wor, neither does the cruise. First thing I did was replace the speed sensor, no luck. I then hooked up my OBDII reader because it also reads mph. As long as the speedo is working, I get an accurate readout , zero readout when the speedo isn't reading. Does this mean the ECM is bad, or could it be somthing else? Everything seems to work fine after the engine has warmed up a little, and stays working until you shut it off and let it sit for a while.