Those are symptoms of low fuel level inside the float bowl. If loosening the gas cap cures the problem, bingo...you'd know that the breather is clogged. That said, there are a number of things that can, potentially, restrict fuel flow: clogged inlet screen, dirt in the tank, pinched fuel hose, damaged fuel hose (they can swell internally), low float level. Based on your results, I'd focus on the fuel system. Ignition problems don't follow the pattern you've laid out.
BTW, the 1.5 turns out from seated is just a starting point for the pilot (idle) airbleed adjusting screw. This controls the amount of air entering the pilot (idle) fuel circuit. Get the engine up to temp, then turn that screw in either direction, as needed, to obtain the fastest idle speed. From there, I usually go ~1/8 turn clockwise to get the idle mixture ever-so-slightly on the rich side. That helps cold starting and off-idle throttle response.