I don’t think that you needed that ECM in the first place.
P0500 indicates a problem with Vehicle Speed Sensor A or its circuitry.
This can cause problems with engine performance, transmission performance, and with the speedometer.
P0720 indicates a problem with the Output Speed Sensor or its circuitry.
This problem relates to the transmission’s electronic controls.
If your car is “driving crazy”, it is very possible that the transmission is in “limp home” mode as a result of bad inputs from one or both sensors.
I am guessing that the supposed need for a new dash cluster was because the speedometer wasn’t working. Am I correct? Since a failure of VSS-A would cause problems with the speedometer, as well as with engine and transmission performance, this further supports my theory that you never needed a new ECM (or dash cluster).
I would suggest that you take the car to an independent transmission shop for evaluation.
Both of these sensors are connected to the transmission case, so this is an area of expertise for a good trans shop. However, you want a GOOD trans shop, so I strongly suggest that you avoid the likes of AAMCO, Lee Myles, Cottman, Mr. Transmission, or any other chain operation. Seek out an indy trans shop that has been in business for at least 3 years, with a good reputation.
And, I would suggest that you avoid that dealership in the future.