take it to a shop equipped with a gas analyzer and get the mixture properly checked and adjusted, preferably one familiar with porsche and with the 3 mm hex tool needed for the adjustment at the fuel distributor..end the guessing game over your mixture condition, then you will have a solid baseline on this parameter for future troubleshooting. if you aren't much of a diy type, get the basic idle and timing set too. your car will thank you for it.
good luck!
ryan