'98 ML320 - EGR tube cleaned but still P0400

[arkie]

I have been living with the problem for a couple of months - but I want to fix it before I go out of town in two weeks. CEL came on in August, took to AutoZone and read a code of P0400 - EGR flow. I removed the EGR valve and tube (nut on tube was frozen so I had to remove the who thing loosen nut out of the vehicle). The EGR tube was completely clogged up, I cleaned it and re-installed the whole thing. Went back to AutoZone and had them clear the code. Clearing the code lasted two days, and it came back on (P0400), I had it cleared again and it came back on a day later. I'm puzzled. Any ideas? I'm thinking a bad EGR valve would have triggered the code instantly. Tube shouldn't have clogged back up that quickly. No other known problems with engine - the only other code I have ever found was the one generated when the engine is left running during refueling (don't remember the code number).