EGR and rough running 30-50 mph

I have a 2000, V6 automatic with 129,000 miles. Several months ago, it started running rough at driving speeds from 30-50 mph and around 1200-1500 RPM (having no fault codes active or inactive) I did a general tune up with new plugs (motorcraft), wires, all four O2 sensors, TPS, and eventually the coil. No change. Checked for vacuum leaks and all the hoses are fine and hold a suction when tested, so on a lark I disconnected the EGR valve from the vacuum system and capped off the end. I did get a check engine light as expected, but he car runs perfectly smooth at all speeds with no vibration or stumble. So after consulting here, I replaced the differential pressure sensor and reconnected the EGR valve and the roughness returned, so I then on a lark replaced the EGR Valve (still rough) then finally the Vacuum Solenoid (still rough). I did reset the controller between each of these changes and the problem is still there when the EGR is in the system but everything is fine with the EGR disabled. Does anyone have any clue as to what I can try next? I can live with the check engine light myself, but plan on giving this car to my daughter and want it 100% when she starts driving this spring. I am an automotive engine engineer myself (diesel though) and suspect some kind of "in range" failure that is just not showing up on internal diagnostics but I am at a loss. Any help would be appreciated. Thanks.

Re: d.p.f.e.

the d.p.f.e has already been replaced. Thats what I referred to as the differential pressure sensor. EVERYTHING has been replaced.....just freaking weird. I am about to pull the EGR Tube and thoroughly clean it out as a last ditch try because that is the only part of the EGR system left to check. I can never run into a straightforward problem....always has to be the weird ones.

I gave up

I finally gave up on fixing it and removed the EGR vacuum hose and capped it off. I have a check engine light, but the car runs smooth as silk.

Now the Check Engine Light Went out

Now after driving awhile with the EGR control disconnected, the check engine light went out...checked at key on and it isn't burned out......so all is well, no EGR, no rough running and now no light

Problem found

Freaking cracked EGR Tube on the bottom side where it can't be seen from the top........what are the odds of that???? Oh well, problem fixed....all new EGR $ystem Par$ for no reason. Also explains how there was flow with the EGR valve disconnected.....