I just bought a 2001 Isuzu Rodeo and it has a problem with shifting from 1st gear.(Auto) I can drive up to 35MPH then it shuts down like it is in neutral until it reaches about 15MPH then you can drive it in first again. I have checked all the fluids, and the transmission oil, it smells clean and looks clean. I have also cleaned the terminals in the transmission mode sensor that sits on the drivers side transmission shifter. Any suggestions as to what the problem might be?

Thanks