This sounds like a transmission problem. If it were a timer issue, the machine would not agitate at all. This is a mechanical failure within the transmission or possibly a worn drive bell under the agitator. On this machine, the agitator is held on by pressure, so take a leather belt and slip it under the center of the base of the agitator and yank both ends upward sharply, holding your face to one side. The agitator should pop right up.
Let the machine fill to the lowest water level and watch the plastic bell under the agitator, even holding it to apply pressure as a load of clothes would. Sometimes the plastic splines under the bell will become stripped, especially if you are bad about overloading the machine. If this is the case, the drive bell (#14) can be ordered using your model number. There's a single nut holding the bell to the agitator shaft.