7.3L Backpressure valve issues

[Paulliewog]

7.3L E450 2002, back pressure valve comes on while accelerating. It is sometimes intermittent, and you can hear it!! On and off, on and off. Sometimes it will come on and stay on at the most undesirable times when I need the truck to accelerate. It is not a seized valve as it does move freely.
Any ideas??