It just occurred to me to wonder what sort of shutter speeds you wind up with? I might suspect some of the focus problem working hand held could be camera motion given what I recall of my exposures; ISO 6 or whatever is not perzactly fast. Of course, I was using a tripod. Maybe a couple of test shots with a solidly anchored camera to isolate variables would be useful. Shoot a yardstick angled at 45º to the lens axis; focus at the middle and see where the results fall.
It does feel sort of perverse that the film that seems to need the most experimentation costs substantially more than the ordinary stuff!
Bright sun or sun filtering through leaves I can shoot handheld f/3.5 at 1/60 or 1/30 if I feel I need more. I have shot sharp results handheld at 1/15th though. I've been getting away with around ISO 8, but lately I haven't even been metering, just shooting at 1/60 and wide open for the most part.