Your mechanic is not gonna do any better then you are and he;s just gonna charge you for his time. try cleaning the IAC and the MAS first....
Driveability problems like this one cost a ton of $$$ to diagnose, and then when they "fix" it it usally ends up being something else and comes back, they drive ot for like 2 miles, it doesnt happen again, and they call it fixed, take your money, and send you out the door only to have the problem happen again down the road....
just my experience, I would rather know why the problem is there myself then have the flippin mechanic tell me whatever he wants.......