Version of Premiere?
Mac OS X 10.6.8
2009 Mac Pro 8-core, Nehalem
Also, I just noticed that different sequence in my project has no issues with keeping the playback resolution I set.
But the sequence that is giving me trouble is populated by two clips which I rendered out of Premiere as Mpeg2. Earlier I had split a really long clip into 2 because the timeline was slow to react.