I don't believe the spots are the fault of the film. I suspect your water source first, or undissolved chemicals. The spots appear to be tiny flakes of some sort too often found in unfiltered water.I don't think the acid stop bath caused the problem, but there is no need to use it except to stop development more quickly. I have used plain water stop bath for years with no problems.
You might try using distilled water for your final rinse.