It sounds like boost cut to me at those RPM's and that cold of atmospheric temperature. This is normal for really cold weather. As it gets colder, the air becomes more dense. Your intercooler will also become more efficient. The more dense your turbocharged air becomes, the more fuel your car will have to deliver in order to maintain optimum air-to-fuel ratios.
What I believe you have experienced is boost cut. This means that your turbocharged air is so dense that your fuel pump cannot properly keep up. The ECU sees this in the form of a spiking AFR (you're leaning out). As it detects this, it opens the wastegate to slow your turbo's boost production. It's at this point that you're feeling a loss of boost.