Falsing in grass sounds like a possible coil issue. Maybe the shield wire or Faraday shield around the coil is flaky. Can you swap in a different coil?
Otherwise, some Land Stars and 505 machines seem more high-strung than others. This may result from varied factory tuning or perhaps the manufacture date is different, meaning BH may have changed or upgraded some of them internally over the years.
Generally, more sensitive is better, but it can also be a nuisance in some places.
Beyond that, was there anything in the area, like a WiFi spot, your cell phone, powerlines, etc. that might explain some of the issues?
-Ed