I would definitely be looking for a physical cause for these changed behaviours, particularly due to the excessive hunger accompanied by the lack of weight gain. Do the vet checkups include a complete blood panel?
As a cat gets older it is recommended that they get a complete senior's blood panel annually. Explain the symptoms and behaviours to the vet and make sure that the blood work includes a thyroid specific T4 test for thyroid function. Although there may be other physical causes for these symptoms, one possible cause that comes to mind is hyperthyroidism.
Perhaps others will have other suggestions. Do keep us updated on how things are going and the results of the vet visit.
"The animals share with us the privilege of having a soul." -Pythagoras
"The soul is the same in all living creatures, although the body of each is different." -Hippocrates
"Let us be loving, hopeful and optimistic. And we’ll change the world." -Jack Layton
"Be the change you want to see in the world" -Gandhi
Kitties: Punky (13), Tweeky (6), and Sassy (7)
1991 to May 24, 2009