Why does my cake fall when it comes out of the oven? Doesn't matter if it is made from "scratch" or a mix It looks great when it comes out of the oven but then falls.
If the recipe isn't the problem, I would say the oven temp is likely the problem. Have you used a thermometer to check the accuracy of the oven's gague?
Where do you live? If you're baking at an altitude greater than 2000', you'll need to make adjustments for it. And I agree with Christine, that you may be underbaking it. Just because a timer goes off doesn't always mean that something is done.
With a screen name like yours, high altitude could be a major problem. If that's just a whimsical, and not descriptive name, this probably isn't your problem. But Just In Case!
There are two separate things going on when you bake a cake: Gasses form and expand, and The liquid batter sets. We want those to line up so that the cake sets just as the most leavening is achieved. In high altitudes, low air pressure speeds up all of the gas expansion, but the batter doesn't set any sooner. When a cake falls, it's because all those gasses are holding up the structure have escaped before the proteins in the cake can support their own weight. We have a few tools at our disposal to try and get them back in sync.
•reduce baking powder/soda
•increase temp and/or bake time
By decreasing the leavening agent, you'll slow the rise of your cake. That means that your batter will be reaching its full height later, closer to when the cake is firming up and ready to stand up on its own. Increasing the liquids may seem counter-intuitive, but in low pressure, water boils at a lower temperature. That means that (compared to sea level) all of your water will have converted to steam much earlier in the baking time, again, long before your cake's structure has formed. By adding more liquid, you can stretch the period during which steam aerates the cake. The increased evaporation at high elevations will also strengthen the concentration of sugar, which harms the structure of a light, fluffy cake. Finally, increasing the baking temp or time will essentially speed your cake proteins to setting.
Tweaking all of these can some times take you too far in the other direction. Depending on your specific elevation, you need to find the right balance.
In adjusting for altitude, you also need to increase the protein in the form of additional flour and eggs, which further stabilize the structure surrounding the tiny pockets of carbon dioxide. The percentage decrease in baking powder (never decrease baking soda, as it is proportional to the amount of acid present; if you decrease it, your result will be very heavy), and increases in protein are relative your elevation. Where do you live?
I'm not nearly as knowledgable about the science involved in high altitude baking, but I live in a city that's about a mile high, and have had some practice (in the gazillion years category). I generally cut baking soda and baking powder in half, and proceed with the recipe instructions after that. While baking cakes and quick breads and the like may be a bit trickier at high altitude, yeast baking is not. Yeasted breads and cakes rise splendidly -- though, of course, you need to check for doubling as you go.