Algorithm convergence problem in DBPSK2?

I did some test on DBPSK2 checked out from gnuradio.git next and
observed some interesting result.

I set up a transmitter and a receiver, using UHD. Two radios were close
to to each other (2~3 feet). I set TX and RX gain both as 18dB. When I
started the transmitter first and then the receiver, I could receive
data packets correctly; however, when I started the receiver first, most
time I could not receive data at all and the CPU consumption soared.

There seems to exist some algorithm convergence problem in the second