Latency problem with a particular session
 

I have a relatively small project with about 16 live audio tracks, no virtual instruments. I'm trying to overdub a DI guitar. No matter how I set the buffers, delay compensation, etc. I always get about a 100ms delay on hearing a live input into a track, even with the track not playing.

However, when I open a new session and add one track, there is no significant latency.

I usually don't have this problem, but I normally am working on sessions from the start. This particular session was raw audio files (BWF) imported into Pro Tools from a Digital Performer session. Could that have something to do with it?

Any ideas?

Thanks!

System:
MacOSX 10.9.5 Mavericks
PT 12.4
Mac Mini Quad (2.6ghz I believe)
RME UFX interface (USB)