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This is a new phenomenon in my DP 9.57. Using 3 instances of play 6.1, Mach5 v3, Arturia 2600, Arturia Proph V, Mini V3, NI Battery 4. Play is new (haven't used V6 before). It seemed to work well 2 days ago, but it's possible I hadn't yet edited MIDI with Play. In any case my dp sequence works fine until I open the MIDI editor window at which point dp gives spinning beach ball and I must force quit & reopen. It doesn't seem to matter whether I restart my Mac Pro (Sierra 10.12.6, 3.2 GHz, 32 GB, Mac SSD startup drive).
I'm running a video (QT) from which I extracted the audio and transferred it to an audio track within the sequence. It seems to run fine and keeps sync with the few MIDI tracks which I've managed to record, until I open the MIDI editor window.
I tried cleaning up bundles, removing anything I didn't need to work with in this sequence, but so far I can't edit MIDI data. I opened another project which uses MIDI tracks (Arturia Mini and Mach 5). This project works as expected.
Has anyone encountered this issue and is able to offer possible solution. Many thanks, jmc.

Following up on my post yesterday and many frustrating hours of trying to 'fix' the sequence that was crashing: the faulty sequences were built from one bad one. I had to build a completely new sequence. So far no crashes using what seems to be the same configuration. Something must be different, but I don't know what. The more I investigated on line, it became apparent that there are not many users who have been having problems with Play 6 & DP 9.5, so I figured it must be me '>}