Need help on 1997 chevy lumina 3.1

My lumina hesitates at speed of 53 to 56 .Already been to the dealer and they can't find the problem.The only code that came up was for the egr valve.Replaced egr valve,fuel filter,plugs,wires,ck fuel pressure,remove
and cleaned trottle body and carbon on trottle body to egr passage.Ck trans,ok.It runs great at any other speed.I don't know what else to do.
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Did you clean the EGR ports in the head? They are the cause most of the time.

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