Not completely familiar with this engine but it sounds like you have a dirty fuel injector or possibly a heat related failure of the injector. I am guessing that the car has individual injectors for each cylinder. Individual coils for each cylinder with two spark plugs each cylinder. What I think is happening the injector is not closing off completely allowing fuel to leak by when the injector is closed. This will enrichen the mixture. If the leak is bad enough it will foul the spark plugs for that cylinder.
First pull one plug from each cylinder after the car misfires at idle for a couple of minutes. This will cause the problem to be seen as a dark spark plug center electrode. The center electrode from a good running cylinder will be white to a very light brown in color. If you see a dark plug pull the other spark plug from that cylinder to see if maybe the spark is not getting to both plugs. Look for one (or more than one) that is darker than the other plugs and there is you're problem. I would try to move the injector around to another cylinder and see if the problem follows the injector. If only one injector is running rich then you might be able to get away with one new injector and a new fuel filter. If more than one injector is bad replace the fuel filter and the injectors. New fuel filters are cheaper than fuel injection problems. Be sure to replace the fuel filter and run the car before replacing injectors. You do not want any prior problems getting into new parts.
If the problem stays with the one cylinder run a compression test to see if you have low compression on the problem cylinder. Probably not because the problem comes and goes away. Low compression problems will stay there.
If the car is running rich on one cylinder it tries to lean out the other cylinders to compensate for the rich cylinder. Causing other problems. Good luck finding the problem. John