Bad idle? mechanic shops cant even figure it out. HELP?

2006 subaru impreza wrx sti with 50k miles. whenever i turn on the car the idle dips very low to almost stalling point for about 5-10 seconds then itll go back up to normal. my car also feels like it is misfiring sometimes when i drive it. the idle goes up and down sometimes but usually is good other than the low idle dip startup issue. i brought it to a couple shops and they both cant figure out what the problem is. they just say to keep driving on it until it gets worse. the car has no check engine lights and i just replaced the sparkplugs and fuel pump due to an issue the car had before this(car would crank but not start). any ideas and advice is greatly appreciated. thanks.