Since we are throwing out possible causes, I'll tell you what mine did. I installed a DAA sight mount on my STI, it had a Serendipity previously. So it went from a 2 sided mount to a 1 sided. Then I installed an RHT thumb rest. This rest used 2 of the mounting holes, so I had the front hole of the sight mount that was tapered and the screw centered the mount on THIS hole only. There was just enough room on the other 2 holes to allow my sight mount to creep up on the back side and rub against the bottom of the mount. This was just enough to slow the slide down and not eject correctly. I would shoot a stage and the gun would work great then the next stage it would be a really expensive single shot!
I took the RHT thumb rest off and put the DAA thumb rest back on, the problem went away. I had my gun apart at least 10 times before I removed the mount and found the problem. Good side was that I polished ALL of the internals, one at a time, to try and fix the issue. Resulted in a very smooth running gun.
There wasn't anything wrong with the DAA mount or the RHT thumb rest, just the loose nut behind the gun!
Remove your mount when the gun gets sluggish, then shoot it without the mount. Might help, might not. Just my .02 worth.
Good luck.
Doug