Not Getting Enough Fuel?

[kbaawv]

I have a 2002 F250 with 127K miles. I am able to accelerate fairly quickly to about 50mph but then it seems to de-fuel. If I back off the throttle, it runs OK untile I put my foot into it. All of this started when I was I pulling 1,200lb utility trailer (unloaded) I was unable to get over 45mph on flat land.

I replaced the fuel filter and it seemed to be a bit better but still is far from fixed.

I think the next step is to check the fuel pressure. Can some point me in the direction on how to check?

Any other ideas?

7.3 with high flow air filter, 4" exhaust, and edge evolution programer.