Q: Engine only runs at 650 or 1300 rpm's, and nothing in between 650 and 1300 or above 1300 rpm's.

Hi there. First check the air filter on the engine. A dirty air filter will starve a diesel engine and prevent it from running correctly. Then check the fuel / water separator and see if there is a drain valve. Drain off some of the fuel in the filter and re-prime the filter to remove the air from the fuel system. If the filter does not have any drain valve, then I recommend replacing the fuel / water separator and the fuel filter. Also, check to see if your vehicle as a block heater and if it does, make sure that its working. Check the glow plug circuit and see if the glow plugs are staying on to heat up the engine. If the glow plugs are not staying on, then check the glow plug relay. Replace the relay if its not clicking or not staying on very long as the timer has failed. If you need further assistance with your engine not starting, then seek out a professional, such as one from Your Mechanic, to help you.