Do you have both a Dynamis and a Lenz system? I'd focus on getting one system or the other working. I take it you have never had either system working properly. I can't say that I have a lot of experience with either.
I can say that it sounds like you have locomotives that are capable of operating either DCC or DC. It also sounds like you are not getting DCC power to your track. When you don't have DCC on the track, a locomotive that can also run on DC will take off at full speed. This is what it sounds like is happening to you.
Someone with experience with either system will need to help get it working properly. Finally, I don't think your decoders are damaged. I would go into CV29 of each loco and disable the DC mode. They won't run on DC when you do this, but they also won't take off at full speed on a DCC layout.
Wiring for DCC