Might sound like an incredibly simple thing, but it could be loose clamp/hose fittings etc. To me, what you've described is once the car is warmer, some boost pressure is leaking a little... which I've been told is more likely to happen when the air is warmer as it can sneak through smaller gaps.
Not sure how true that is, but it did happen on my stock BOV on my 33 GTST. Gasket wasn't sitting properly and had a tiny, TINY gap, so I was loosing pressure through that, but only once the car was at normal warm temperature.