1. did the boost controller spike to anything when it was set at 11 psi previously, or did it stay at 11psi all the way through the band? If it spiked before higher than 11, you probably just need to play with the settings again.
2. Now that it spikes and falls off, what rpm does it spike at, what rpm's is it trailing off? I'm willing to bet it hits at 4k and trails to 7psi after 5.5-7k or so.
See if you can drive it w/o the mbc in series and see what the boost gauge does. (or turn it all the way down)... if it drifts below stock
boost then you know your wastgate is the culprit.
Seems to me its more likely the mbc... they tend to spike, peak at full boost (3.5-4.5k) and slowly drift back down at higher rpm. And you were probably enjoying yourself so much you concentrated on keeping your hands on the wheel rather than stare at the boost gauge to notice until now
I think that weather can affect it a little bit as well... from what I've seen, but I'm not Confusious.