This sounds like the adjustment to the cable has loosened. Here is a very similar problem and solution previously answered on FixYa, and it sounds like this might be your problem:
It could also be a problem with the PWM speed controller. Here is some information about that from fishingandboats.com:
WHAT IS PWM SPEED CONTROL?
The Pulse Width Modulated (PWM) system uses transistors that are rapidly switched on and off, or pulsed. The Motorguide DuraAmp and the Minn Kota Maximiser switches on and off at around 20,000 – 30,000 times per second. The transistor switch on/off times are controlled and the current flow level is dependent on the ratio between the On and Off periods. The larger the ratio, the more current that will flow. The lower the ratio, the less current that will flow. This variable on/off ratio gives a square wave. The PWM is a much more efficient system than dropping resistors or coils. The pulses reach full voltage and produce greater motor torque. The only disadvantage is the generation of electrical interference. As all trolling-motors at switch on with load they can take quite large currents, the Maximizer has a soft start feature which ramps or limits the trolling-motor speed at start up so it is smooth. This helps extend trolling-motor life.
WHAT ABOUT TROLLING-MOTORS MAINTENANCE?
Keep the tolling-motor clean and dry when not in use. Where used in saltwater estuaries all salt must be washed off. Sacrificial zinc or anodes on a saltwater model shaft must be monitored and replaced if required. Make sure that the surfaces are clean, and it is good practice to remove and refasten regularly. The zinc must be fastened just above the lower unit. If the trolling-motor gets a season of hard use, get the trolling-motor checked and vacuum out all carbon dust. Nothing wrecks commutator condition or causes sparking more than excess dust deposits. The dust can seize brushes in brush-holders. The key to DC motor life is regular maintenance.
The motor power is lower than normal
Be careful when using your motor when fishing in thick vegetation. If this wraps around the prop it causes overloads and higher currents. The high currents cause general overheating which affects all parts of the motor. Check for low battery voltages and for loose cable connections that cause resistance and loss of motor power.
The motor has lost one or more speeds
1. Check for loose connections in the speed switch and potentiometer.
2. Check for loose connections within the top housing.
3. Check the rotary switch for burning or other visible damage.
4. Check contact continuity or resistance with a multi-meter or short out each speed terminal to verify the switch contacts. If switch is in good condition one or more speed coils may have failed.
My guess would normally be that the PWM (pulse width modulation) board is not fluctuating as it should, but the above suggestions are from people who are in the business.
What I would also do is call a Marine Service Shop and see what they think...a lot of times they've seen the problem, and can tell you exactly what and how to look for it.
Good luck to you, thanks for using FixYa, and hope this helps!