Curious, I poked around photo websites but did not happen on a specific review, but it does appear this was an early Spiratone relabel, with the distinctive foot. Owners tended to be satisfied.
I came across no nasty owner posts, so I will assume yours has an unknown issue.
I would not expect useless, Spiratone stuff was at least solidly usable, they would have been taken to task for less. Word of mouth was powerful, pre-internet. Some stuff was excellent.
Photo lenses are sometimes disassembly capable, but I am not aware of any adjustments other than spacers shifting the focal point to flange and perhaps element spacing.
My guess is something is loose or contaminated.
Oils from the focuser grease made many diaphrams 'lock up'. Oils could certainly contaminate a mirror with a thin film. On writing this, I think this is what got my first Minolta AF mirror.
One thing you can check is serious contamination internally, degraded mirror coatings, scratches, and, particularly, loose elements or retaining rings.
I picked up a Minolta AF lens that would not focus, and whole disassembling it to fix diaphragm contamination, the rear elements just fell out. Apparently the retaining ring was very loose and not noticed. On reassembly it was very sharp (and very clean).
I have a Minolta mirror that had some kind of contamination on the inner 1/3 of the main mirror, and it showed up as a shadow in the 'bokeh', an out of focus highlight. Gee, sounds similar to Suiter star testing... Point at a bright small light, and look at the out focus donut. It should be smooth and even.
So check visible surfaces for issues, check retaining rings are secure (jiggle and wiggle to see if elements shift; if tightening is needed buy a spanner (but toothpicks are safer than tools if you don't want to spend the $$), and don't overtighten!
These small old mirrors are fun to keep around.
Better ones have multiple elements, but yours may not, in case you attempt disassembly for cleaning. Make pictures of every step and every screw to aid reassembly if you try it. I've never had a mirror photo lense apart.