There could be a few reasons as to why the cake is sinking. One reason could be due to the oven temperature, if you open the oven too many times during the baking process the temperature drops rapidly and the cake will sink. This is mainly because the eggs and leaveners need heat to expand. So keep those oven doors closed until that timer goes off!
Another reason could be due to old eggs or bad baking soda/powder. If your eggs are more than a week old the steam in the oven won't make them rise as well as fresher eggs. If your eggs are fresh but happen to be cold, the steam won't expand them either, it is best to use room temperature eggs. If your eggs aren't the problem check your baking powder and soda. An easy way to do this is to put a little bit in a bowl and add warm water or vinegar. If the mixture bubbles your good to go, if not you should buy new leaveners.
Browning Quickly/Uneven Baking
One reason for this could be the oven temperature. If your oven is set above 350F it could cause the browning of the sugar to happen before the inside of the cake bakes. If the oven is set at 350 and the problem is still occurring, then it is best to either have your oven serviced or to lower the oven to 325.
If your cakes aren't baking evenly it could be because of the location in the oven, it is best to place the cake in the middle of the oven if possible. If you don't have a convection setting on the oven you should rotate your cake halfway through the recommended baking time.
If this is happening when you flip the cake out of the pan or try to move it onto a cake board it is probably because there was not enough egg in the recipe or the cake has been over baked. If it is the former I would suggest adding one more egg to the recipe or adding sour cream. The amount of sour cream should be less than or equal to the amount of butter in the recipe. If your cake is over baked, bake it for the shorter amount of time on the recipe. If it says 20-30 minutes, set the timer for 20 minutes. It is always best to check and have to keep baking rather than over baking.