pulling the cap off and it stalling is normal because the breather system needs to be sealed for it to run, by pulling off the cap ur causing a air leak so thats normal

misfire could be caused by a number of things, do you no which cylinder is not working? a compresion test is worth doing for this fault tho, as you say its got lack of power too

any work been carried out on the car recently? worth checking the fuels lines from the metering unit to the injectors for leaks ect