My 2003 Ford F-250 6.0 diesel is fine when it is cold, but after it gets hot the engine hesitates and losses power. If I press down on the gas is runs fine for a minute. This mainly happens in the 1800-2300rpm range. I took it to a Ford dealer and they replaced the EGR valve and installed an injector pressure control sensor to the tune of $900. The problem appears to be getting worse. What is my next move?