Has the AFM on the 87 been opened up? How does the contacts look like on the plug? How's the gasket between the AFM and the housing that the AFM mounts to? Double check that the charge pipe on the AFM is tight and there isn't any air leaking.
Look for the small things first. If all else fails then you can download the FSM in the sticky and troubleshoot your AFM. There are steps in the FSM to verify resistance for each pin to make sure the AFM is within specs.
