Toyota Repair: Code PO401, toyota land cruiser, egr valve

Question
1997 Toyota Land Cruiser.  Has 110000 miles on Engine.  Bought LC with Check Engine light on.  Had vacuum hoses checked and then cleared.  Two days later it was back on.  Back into shop they replaced Modulator
three days later CEL is back on.  Back to shop three weeks ago they replaced VSV and kept the vehicle for another week and drove it on local roads and Interstate to Spokane, WA.  I drove it another 300 miles and CEL stayed off.  This past Sunday drove to town and CEL is back and yes it is PO401 still.  What should a next test be?
Thank you for your help.

Answer
Have the shop check to see if this is a california certified vehicle, if so it will have a EGR gas remp sensor located on the bottom of the EGR valve casting, these sensors can fail at times. If that's not the problem maybe replacing the EGR valve would help.