Coffee Cake Recipe Help

[tabby1881]

My aunt passed away several years ago. We all loved her coffee cake. Of course, being me, I've misplaced her recipe. The recipe is not difficult, but I'm having trouble with it. I think the recipe is fairly common. Some of you might be familiar with it, or perhaps you can tell me what I'm doing wrong. The recipe uses 1 box Duncan Hines Yellow cake mix, 1 cup sour cream, 1 box instant vanilla pudding, 4 eggs, oil, vanilla. I'm confident of my measurements, it is the mixing that I'm having trouble with. The only measurement that I messed up with is the pudding mix. I bought a small 1 oz box instead of a 3 oz box. Would that make a difference? When I take the cake out of the oven, it looks fine. A few minutes later, it looks like someone sat on it. There is a layer that is thick & spongy. KInd of hard to describe. I use a tube pan, 350 oven for 55-60 minutes. The mixing is what I'm worried about. The recipe online that I found to be most similar to what I remember says to mix until smooth. Other recipes that look similar say to mix 4 mins or even 5 mins. (I mixed for 2 minutes) Would over or under mixing cause the cake to collapse and have a spongy (looks like foam pillow) layer? My oven works fine on other cakes.

I'm so confused. I did make the cake about a month ago and it was perfect. Last two cakes have not worked out. I'm determined to get this right. So, is it the wrong size pudding mix, or is it my mixing? Bake it longer?
Thank you.