Good Answer Jack!! We also work on sites with mixed materials piping. Some of the standard pre-cuts, whether in steel or PVC have little or no quality control. We have found that "chasing" the threads with the appropriate die solves most issues and allows for a more uniform "plastic deformation" of the threads. Again, the key criterea is that you have only 3 (THREE!!) threads of engagement in hand tightening before tool use for the final seal.
A last resort trick that we sometimes use is Gas Line Teflon Tape. This is colored YELLOW (most likely for the ease of the inspectors) and is about 1 1/2 times as thick as the standard white. Sounds like a good match with your low pressure requirement.
Any chance there is a shock or vibration issue that is 'working' your connections loose??
Best on 'ya
“Any sufficiently advanced technology is indistinguishable from magic.” Sir Arthur Charles Clarke
"Success is not measured by what you accomplish but by the opposition you have encountered, and the courage with which you have maintained the struggle against overwhelming odds." -- Orison Swett Marden