Check Engine/ERG Code

I'm a happy owner of a 2002 F150 with a 4.2 V6. At 86,000 miles, my Check Engine light came on and the code revealed an EGR valve problem. I replaced the EGR valve and erased the fault. Everything seems fine for about a day and the light came back on. I rescanned the system and the same code came up. What could be wrong? I've also noticed the gas mileage has also dropped by miles.