2006 ISX CM870 High Pyro, surging under load

[Kbarnes0]

I'm going to join in on this one as it states almost identical to what I was going to post. 06 CM870 high pyro above 1500rpm. I don't have a surge. This load gross weight final put in enough load for me identify that it pulls really well at 1450-1500rpm and maintains steady egts. As soon as you drop a gear if it's over 1500 you really got to back out and feather it.
Mine is demandated with a tune from a group member. Aftermarket boost/ Egt gauge. prob is pre turbo. Boost range is smooth and peak I've seen is 35/36. Typically 1700rpm/1200pyro would be 25-27psi. Not sure if it should be able to operate towards max boost pulling hills, but you can't get anywhere up there, so I just back out of it and go slow.
I was thinking high pressure pump not keeping it fed?
Hopefully we can identify the cause. I will work through a few of the previous mentioned items as well.
Does the cm870 have the command fuel pressure in insite? Or just rail