The problem may be with the EGR valve or carbon build-up in the engine. Carbon bits can work their way loose when the engine is under heavy load - climbing hills. The carbon can stick in the EGR valve holding it open. The valve may or may not clear itself out. Performing a "top engine decarbonization" procedure may help or correct this condition. Any good repair shop should be able to perform this procedure, It may be a bit complicated if you have never done it before. If the engine decarbonization does not correct the problem you may need to replace the EGR valve.