i will go for the ICV ..my car was doing the same, over time it got worse and ended throwing the check engine light up . i tryed cleaning it aswell (you did take the hole icv unit off yeh and not just the cap) and it did not word..
get hunting iff its is mate thay dont come cheep. about £260 from subaru.
have you had a look to see iff these any faults held on the ecu?