I've heard this story before. Very first thing I'd look for is some kind of animal nest in the airbox obstructing it, if you haven't already. I used to have a lot of customers with cars they didn't use for extended periods of time, it wasn't unusual to find a motor's airway completely hogged with bird nests, mouse nests, packed with acorns from some rodent, mud dauber nests, etc, even when parked in a garage.
For some reason, critters LOVE airboxes.