I have an 01 F-550. When going up hills RPM and speed slowly down down to almost to dead stop. Feathering accelerator does nothing. Changed fuel filter, oil and filter, replaced fuel pump due to low pressure. Install fuel pressure gauge at right front head. When going up hill pressure drops to 0PSI but vehicle still runs. Service engine soon light comes on with code P1211. Shut key off for 30 seconds to 1 minute. Start up again & it takes off. ICP is 6 months old. Cam sensor less then 1 year old. Right side injectors only 3 months old. Any ideas?

 

Do you have the Steel fuel tank mounted behind the axle? If so, drop it and I will bet you the coating in the tank has failed, clogging up the pick up screen in the tank. Common problem on the cab chassy trucks with the steel tank. We have seen 2 of them do this.

 

I second the opinion of an intank issue, fits your symptoms to a T since you already changed the filter.