99 eclipse shuts off when I let off the gas?

does the engine light or any other indicator lights flash or come on when it happens? you should get a programmer/code reader. newer auto transmissions are very eletronic its not a 71 chevelle where you can drop and go, your car's computer still thinks that you have an auto trans so look into flashing your ecu hopefully that will solve the problem once the car realises that it got a better trans!