Thanks to everyone for taking time to comment. It's all valuable.
AWAP & paulwlee - I think you two have nailed it. Those comments you made were especially helpful.
Right around the time I did the original post, I did a forum search on the phrase "over breathing" and got a page of results. I turned my radar on and went looking for entries from:
- DA Aquamaster
- AWAP, and...
- Oxyhacker (I've just purchased & read Oxyhacker's (Vance Harlow's) 252 page book about regulator maintenance - its' a real page turner! sorry, sorry, just getting silly. It's a great resource - I highly recommend it, really).
Anyway, in one of the threads, DA Aquamaster mentioned that he had experienced this 'over breathing' thing under circumstances that were roughly similar to my situation (instead of chasing something, he was swimming against current). He went on to say that the problem was due to the max flow rate of the regulator he was using - an MK3. He quoted the flow rate of the MK3 and then went on with a discussion of other Scubapro regs/flow rates that would have solved the problem.
And now I see AWAP's comments herein that seem to mirror both DA Aquamaster's experience and my experience. And, while AWAP didn't say that the MK20 has a higher flow rate than the MK2, I think that that is what he's referring to (of course, there are other differences besides flow rate).
And PAULWLEE, who has had the same reg as I currently have and who has moved up to an Atomic, points out how he has discovered that the venerated SE2 just doesn't compare - performance wise - with more contemporary products.
I can find flow rate specs for MK2, MK3, MK10, MK11 (there's lots of info readily available on Scubapro) - but I can't find them for my SE2. I've checked the current AquaLung Conshelf Technical Maintenance Manual, I've checked scubadiving.com, I've googled for it. Lots of people have something to say about the Conshelf - just nothing about max flow rates of the SE2. So, that alone makes it hard to precisely correlate my situation to the equipment used by AWAP / DA Aquamaster. But I'm guessing that I'm experiencing the same thing that AWAP & DA Aquamaster have experienced.
It's all good. One more reason to treat myself to an upgrade! Thanks again.