What you describe is exactly how my factory HG's acted when they first started to go. I drive it easy, all is good. Work the motor a bit and pressure would rise quickly. I drove it for 6 months before pulling the heads because it progressively got worse. Even the day I tore it down if boost stayed below 25psi, then all was good, anything above that and pressure would rise. Currently mine is at 3psi when I start the truck up, drive it 10 miles and get the temps up and I have 5-7psi. Run i hard and it will climb to 11 or 12, but as soon as I get off the throttle it immediately drops back to 7-8. Even towing heavy I never see more than 10-12 at the max. I'm still betting it's HG's, but I do hope it's something a lot simpler.
When the factory hg let go on this truck..10psi of boost it puked ccolant.. This is just weird situation going on here now. I'm just wondering how long exhaust gas stays in the coolant from a blown headgaaket.. I'm wondering if its possible that they are only leaking under WOT? If that's the case..the gas test should have detected the exhaust in the coolant??