Minolta and Pentax slr cameras are known to suffer problems in the lower gear train (under the bottom plate) which will cause this sort of thing. Essentially one gear in the train does not spin freely enough on the shaft and this will cause the mirror to hang up (especially on slower speeds). Maybe 15 years ago I was receiving so many questions about this on the Minolta SRT series that I made a set of annotated photos I scanned into image files. A French user put the answer for the Spotmatic on YouTube, and it is still there. Good luck.