Update: That sticky shutter on a Pentax K1000 took me 3 days to fix, not the hour I thought.
It was just one tiny spring behind the mirror box that had lost its tension. I kept cleaning the curtain tracks and checking the speeds, but the real issue was hidden. I only found it after taking the whole top plate off for the third time. Anyone have a good source for those specific little springs, or a trick to retension them?