06' 379 w/isx 475 turbo boost issue

i drive a 06 379exhd with a cummins isx 475 and eaton 18spd. the problem im having is at random times when im pulling a hill the turbo will spool to full boost and then just drop to about 20 psi and will surge from 20psi to 25psi then whenever it feels like it, it will go right back up to normal boost pressure and run fine. there is no rhyme or reason when it does it. i have noticed when this happens the pyro goes way up also. then last week it stared to doing it again ,but this time it will only do it if the CEL is off, but when it comes on (the CEL that is) it runs pefect. but like i said it happens when ever it feels like it sometime all day long it will surge when boosting,other days it runns perfect. what should i start checking?. hopefully this isnt to vague. thanks for the help!