Persistent P0401 Insufficient EGR Flow Code on '95

Happy Thanksgiving,

I have been chasing a P0401 code on my 1995 Riviera for 9 months.  I have replaced the MAF sensor, EGR, EGR connector, PCV, Vacuum Harness, and Smoke-Checked for vacuum leaks several times.  I have plenty of vacuum at the EGR mount, so I'm certain the EGR path is not blocked. (I have not removed the Throttle Body to clean the ports.)  Nonetheless, the code returns every 80 to 200 miles, with no accompanying codes.  Using premium gas seems to get me longer periods between codes.  I have read everything on the internet about the problem.  Anyway, I would appreciate any thoughts on what to try next.  Regards.

Start the car and watch the tach while in park.  You can drive a bit to warm it up to op temp. When warmed, park car and let run.  Watch the idle.  

If the tach wanders or flutters or hunts, remove and clean the throttle body.  Reason for the hunting is buildup in the TB, most likely the EGR and-or IAC circuits, in my experiences. If you follow the piping from the EGR you'll see it ends up there.

To clean throttle body you'll need coating safe TB cleaner and a metal pan. And a replacement gasket.

More details? post your questions.