Well, if you were getting EGR codes, you should have replaced the EGR valve right away, or at least cleaned it. I just did mine and it was $150 for the part, codes gone.

Are you getting a Check Engine Light?

Now, I am not 100% sure, but I have heard that if your head gasket is leaking you might have a milky residue in your oil (coolant leaking into crankcase). So, check your dipstick and make sure your oil is full and looking normal.

Bart