In X3 I have a midi track set to an outboard synth. I switch the track between several synths demoing voices. When I play it thru one specific synth the entire track volume diminishes to a lower level until I hit STOP. Then I hear a sharp rise in the volume output.
 
I drag copied the midi data to a new track and assign it to the same synth with the same settings and the new track will play at a louder volume consistently with no rise when stopped... for a while. There are no automations or controllers on any tracks.
 
What can be causing this?
 
Thanks,
Steve

I suspect that you do have controller information on the track, even if you didn't put it there. I've been bitten by that a couple times and the symptoms were very similar to what you describe. IF you change a fader level on the track prior to playback does it remain where you put it when you start playback? That was my clue and it took a while to find an automation node way down the timeline. I didn't put it there, total mystery.

There are definitely no controllers here. The weird thing is, after soloing another synth for a while and then soloing this track, the volume will increase but then fall back again. I am suspect of the unit itself. It is old and probably starting to fail as it is currently the only synth having this issue.

Started a new project with fewer midi & audio tracks and the synth now seems to work fine. Problem was more than likely a corrupted project file undergoing a 1000+ edits and changes causing the synth to not output fully. I can't really blame the midi clip as I imported the original clip into the new project.
 
Keeping a project as simple and uncluttered as possible seems to make things work much better.