GnuRadio CC Decoder or How to use FEC Extended Decoder
Inside CCSDS channel coding standard, there is a synchronization word called the ASM.
It is a pattern sequence of 32, 64 or 256 bits which is used for synchronization inside the stream. It is usefull to solve the two points I describe below.
Inside your GNU block diagram I see two things that might be an issue:

Phase ambiguity: There is no mechanism to solve the phase ambiguity of the BPSK in demodulation. I am not really sure if Viterbi decoder works fine if not solved (the ambiguity).

Two bits ambiguity of CC: This type of code with a rate of 1/2 produces two encoded bits for one noncoded. At the input of decoder, you must be synchronized with the first encoded bit of a couple.


This could explain that sometimes your system works after switching on/off. On chance over two or maybe four...
Other remark, using an alternating pattern of 0 and 1 could make you miss something debuging. Try a 32 bits pattern.
Tell us if you found something