Pete 379 sputters at idle when loaded

So I’ve been fighting with this the past week, I got a 1994 peterbilt 379 dump truck with a 3406b. Top end has been rebuilt about a year ago and has only been driven once in the last 6-8 months before I started driving it last week. So to begin with, oil pressure is lower than what it should be but I’m still in minimum specs which is 25psi. But this is the weird part, when I get loaded to 54k pounds, let me remind you I’m still under the loading bay and haven’t moved or anything trucks still in neutral and all. As soon as I get loaded, my truck starts sputtering, rpm’s drop below 5 and oil pressure drops below 20 and just bounces around there but goes away and acts fine when I get on the accelerator. Hooked up a mechanical gauge and it reads the same. I do have a leak in the airbags on my drives, also my bags don’t air up until I start driving even if my pressure is built up all the way. But I’m not sure if that has anything to do with my oil psi and rpm’s dropping when I get loaded. It just doesn’t make sense to me, it only does this immediately after I get loaded but it’s perfectly fine when I’m not loaded. Oil and filters been changed along with fuel filters too. Neither was contaminated. It only does this at idle. I also want to mention that I have other trucks running about the same oil psi and they don’t do anything funny when I’m loaded.