3406 power loss in after five minutes driving

we have a 3406 in a peterbilt. i think it is a 1995 and it has 1,000,000+ miles on it.

last year we did new injectors and sleeves. they were all cracked.

if the truck idles it builds about 20 psi boost and has good power. once the truck starts to roll after about five minutes of drive time it drops to about 12 psi boost and loses all its power.

it has new filters air and fuel. it has a new fuel tank vent, but had gone a long time without one.

any thoughts or suggestions?

 

No fuel pressure guage.

How do I check it?

I don't think it has a lift pump.

 

the tag says just "3406" there are no letters after it. the serial number is 05ek86190.

i have repair order that the following work was done 18 months and about 10,000 miles ago:

replaced fuel injectors
replaced injector sleeves
set injector height
replaced turbo
replaced fuel transfer pump
replaced air to air exchanger

we have only visually checked the fuel lines. how else do we check them? how do we check them under a vacuum?

this is a heavy equipment transport truck so for weight purposes it has only one tank on passenger side.

we have not looked for a tank filter that i know of.

we have just replaced the tank vent. the old one was bad. we did have the truck die on us once because of the failed vent. could this have caused fuel lines to collapse and fail as well?