Look at your timing when coasting vs the timing when you are idling... your coasting timing should be slightly lower than the idle timing and the air going through it should be higher than idle. The purpose is to create enough torque reserve so idle routines can easily bring it back to idle.

With all idle routines shut off (no iac adjustments and no timing adjustments) you want your car to naturally idle 200 rpms higher than your commanded, and then let your idle routines bring it down. The more unstable the idle is, (more reversion caused by overlap) the more torque reserve you need to achieve proper deceleration/coming back to idle and idle stability. This is contrary to what most people recommend which they want you to adjust your timing so that you get the most vacuum, thus removing ALL torque reserve from the routine and thus extremely unstable because now no timing changes make any difference in the torque output of the motor that is idling.