MY KITTY IS SICK.... please help?

I have a single male cat that is almost 2 (i have reptiles as well). The problem that i am having is that Eli (cat) keeps getting sick all over my floor, window, bed ect- and its also being accompinied by Diarrhea. It all started when I changed his food from purina kitten to nine lives. He wouldnt eat this and i stopped using this because i thought this was making him sick. I have since changed it to whiskas which is what we always used as a "run out" substatute. However the symptoms are still present. The odd thing is that they come and go. Hell get sick for a day or two then be fine for a week or two. I have called a vet but they said he sounds fine... cuz its not contious????? He is up to date on his shot, does not have a hairball and NO HE DID NOT EAT ANY OF THE REPTILES! please help - ps dont just say take him to the vet-