2003 Mitsubishi Galant EGR Insufficient Flow P0403

I've had this code come up three times since I've had the car which has been about 5 months. When I got it the light was on. After replacing EGR valve and solenoid the engine light went off. Then few months later it came on again. After not taking action for a week or so, it began idling roughly. I rechecked the Egr valve, checked the wire connection to the solenoid, attempted to clean egr pipe of carbon deposits (didn't seem to be anything as far as I could go) and inspected the vacuum hoses. After taking these actions and replacing solenoid a second time, in July of 2015, the light went off and car was running fine. As of August about two weeks ago, though it has not idled roughly, the engine light reappeared with the same p0403 code. What else could it be?