Is it convenient to try a different TIU channel and see if same results?
Is this something that just happened recently, or just after you changed anything?
What is your power source? I assume this is not passive mode?
How big is your layout? Is is a "simple" loop?
Have you tried the Protocast mode to play music or whatever as an engine goes around a loop? It's an alternative way to narrow down to very small sections of track for problems.