cupcakes keep on sinking in center
Oven temperature gauges are wildly inaccurate.  So it is very likely your times and temperatures are off.   It sounds like the cupcakes are under-cooked, and are sinking as they cool. Use a good digital thermometer to test the internal temperature of your cupcakes.  Shoot for 205 - 209F (96-98C) internal, at the center of the cupcake.  Alternately, you can use the low tech. method of testing them with a toothpick by dipping into the center.  If it comes out clean, they are done.  If there is batter sticking to the toothpick, keep baking.  ...and it doesn't hurt to have a good oven thermometer to see where your oven stands in terms of accuracy.