OP: Make one change at a time. It becomes very difficult to troubleshoot when too many "fixes" are attempted at the same time. May I humbly suggest that you hold off on the Seafoam until after you have addressed the plug issue. This engine is hard on plugs. 52K is going to be probably as much as twice as many miles as you ought to have put on the car without at least having pulled, checked, and regapped them, and it probably beyond their useful life.
At that mileage I'd put in a new set and make sure they are the proper heat range and properly gapped. Only after doing that and still having the problem would I consider Seafoaming, especially if you have already replaced the EGR valve.
If you decide to Seafoam, check with guys here who have done it on this engine. I have not. You know it does nothing on this engine to add it to fuel. The high pressure direct fuel injection system is going to prevent it from doing any good. You are going to have to add it to the intake at a point where it goes into the engine through an intake valve. With direct injection, the intake valves only introduce boosted air, not an air fuel mixture. I think that some guys are maybe putting the Seafoam in through the bpv recirc hose. Check with those who have done it.
Do the plugs only at this point and let us know how it goes. I think there is a very high chance that it will get rid of your momentary stutter.
