Compressor/AD-9 purge problem

So up front, '98 Pete 378 w/Series 60 Detroit. Last spring did a bunch of work, including new compressor/governor/check valves/rebuild AD-9/check lines for leaks. She's not building water or blowing oil, holds air well tho not as well in colder weather. At low idle up to about 1300 rpm, air building/purge works normally. Air system pressures are DOT spec or better for building/retaining. 125psi at 7p is about 30-45psi at 6a the next morning. Startup/warmup first thing at 900rpm or so, normal air build/purge. On the road after that, and all day long, as the pressure bleeds over time with brake and trailer use, the AD unit will abruptly start to blow high pressure air off starting at about 115psi, lasts for about 30 seconds or so. As it hits the cut-in pressure of 105psi, it will abruptly stop and quickly air builds to 125psi, and the AD will purge normally. If you let off the throttle during this bleed down from 115psi, the AD unit will cease it's higher pressure blow off, but you can hear the purge valve kinda fluttering and burping air. This oddball cycle is making my compressor work 3 times as much during the day as it should. I've put two different governors in, first a M*****r, then a H****x. The compressor - new B****x not a reman. Hooked a valve in and checked that my line pressure was what the dash gauges indicated, they're very close. The only thing that isn't new in the loop is the AD-9 body. Fought a few air problems before, but this is a new one. If all the components weren't new, I'd be less puzzled. I did have some contamination in the tanks from the previous compressor that was going south, but switched out cartridges and added a bit of alcohol to the lines/tanks every month, until everything blew clean, both truck and transfer trailer. I'm kinda versed in normal air system problems, but this one's got me and my mechanic stumped.