If your connections at the rails aren't perfect, you can loose power. Generally speaking, if the power dips enough, the DCS signal will suffer. I did read about your Lionel engines running. I'm thinking it maybe your track connections still. Maybe you could try soldering at least one connection as a test to see if things improve.
What gauge wire is at the rail connection?
How many DCS engines are on the same TIU channel?
What is your power supply?
