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