Why does my cake fall in the middle as it cools?

Are you over-folding your egg whites? Or are you folding (not stirring) them? That could be the problem.

I'm in the northeast so elevation is not an issue.This has happened with standard cake recipes and those with egg whites.I use oven thermometer to check convection temp as it sometimes runs hot and am careful not to over or under bake or overmix batter.Still occasionally my cake falls.

W/o knowing details, it's hard to tell. Could be underbaked. (Check your oven temp.) Could be overbeaten. Could be altitude problems.

Betty Crocker's cake FAQ has helpful info:
http://www.bettycrocker.com/products/supermoist-cakes/faq

Lots of cakes sink, for many different reasons. A little deflation is normal but a "well" in the center means something(s) went wrong. Check out what I wrote here, and tell us if you find your answer~ http://eggbeater.typepad.com/shuna/2009/02/why-do-cakes-sink.html

Where do you live? If at an elevation higher than 2000', you should be making adjustments for altitude. If that's the case, please tell me where you live, and I'll give you the adjustments for your elevation. Alternatively, it may not be completely done in the center.

Hard to answer specifically without knowing what kind of cake. An egg white based cake with little or no flour will "fall" as it cools and that is expected. In a different case, the cake batter may have been too much for the pan used.