I just got a gslse a few months back did the whole tune up everyone does when they get a new car a few weeks ago i noticed the idle would go almost to 0 then jump right back to about 700 when i was coming to a stop either on highway or regular street drive then when i went to put it in first gear the same thing now car dies or idle goes to 0 then back up took apart the intake to clean it found oil that looks like tar super sticky in air filter box cleaned it up put everything back together but still have the same problem n oil in the air filter box is back to have no idea what can be causing both problems

 

check your pcv,