WHILE DRIVING MY 04 ESCALADE AT SPEEDS BETWEEN 40 THROUGH 60 MPH. THE RPMS JUMP UP AND DOWN BY 400. WHAT COULD THIS BE? CADILLAC SAID A CRANK SHAFT SENSOR

If this is a points type ignition, check for excess play side to side in the distributor shaft, and also for timing chain slack--easily tested by removing dist. cap and rotating engine back and forth with wrench on crank pulley bolt. If rotor doesn't move immediately as crank turns you have loose timing chain.

Could be bad headlamp relay.

Check battery connections and have battery and charging system tested.

check your crank position sensor.

problem is electronic but exactly what i have no idea without my head under the bonnet ,try the dealer as it may be cheaper in the long run

have you checked the fuel filter , maybe it is plugged

THIS MAY SOUND STRANGE BUT DO YOU FEEL ANY VIBRATION IN THE SEAT WHEN YOU HEAR THE HUMMING IF SO I WOULD SUSPECT THE U/JOINTS IN YOUR DRIVE SHAFT ARE GOING BAD

hav u done your needed maintenance like checking oil level-itz done with engine going in park also shift cable needs ajusting or solenoids on the electronic models if alot of miles then a rebilt is posible am tired and its late-try testing the tps sensor 1st has to with shifting***

if the computer is located in the engine compartment it could be overheating. the solution to the computer overheating is a new computer. expensive. I have seen this happen on other vehicles as well.

If this problem just started I would try to get a jump on it. It will be easier to track down if the light just started coming on. Otherwise other problems may come into play and the mechanic will need longer to sort them out. At around 70.00hr.Cheapest fix is to try and drop in a set of plugs. Good luck.