Holley Red pumps have been used with good results by several of the users in here.

3 PSI is a bit low in my opinion, mine sets at about 6 PSI at idle.

 

You really can't test for a weak injection pump on the truck, you would have to pull it off and have it tested at a local injection service shop. Of course, if it were me, by the time I got it off I would go ahead and have it built.

 

try looking at the return lines i had a similar problem, and one of my original rubber lines were cracking and lesking diesel, wouldnt explain the power loss though, also may want to check the plastic caps, and the injector o rings.