Let's not run down too many rabbit holes here. In his OP, he said that on the second sequence, the channel LED was flashing but the light string was not. The LE was receiving the commands to light the channel but was unable to do so. This clearly indicates a hardware issue beyond the PIC. The possibilities are the opto, the triac, the output plug, or the light string. Since the channel would work sometimes, I really doubt that it is a failed component since when most solid state components fail, they fail and that is it for them.
He claimed that he used a known good string and it still didn't work so it's not the string. I would first check the output plug and make sure that the connection to the terminal block is a good one. With the power off and unplugged, I would also run a continuity test between the terminal block and plug outputs. If vampire plugs are being used, they sometimes have issues making a good connection. The next step would be to reflow the triac connection's. An iron hot enough to solder in a resistor or a cap will not always be hot enough to properly solder in a triac. Triac's have a lot of heat sinking capability and while the solder may flow to the board, there may not be enough heat to flow properly to the pin of the triac. This type of cold join is hard to see and can make or break connection for the smallest of movements and a few degrees of temp change can also make or break it.
The opto can also be an issue but after checking the sockets solder joints, the easy thing to do is to swap it with one in a different socket. If there is a bent pin on the chip, that will be seen when the opto is removed and unless the issue follows the chip, the opto is not the problem.
