series 60 boost

[BradKuz]

Hey fellas, so this is my first go on the forum stuff. Ive got this 07 Freightliner with a DD13 in it. one of the guys in the shop did a head gasket job on it per customers request. Since then he has been back claiming everything under the sun is wrong with the rig "ever since" the head gasket job. The latest is a boost problem. If you stand on the throttle in high gear the boost will come up to about 29-30 psi and then even though you dont lift your foot the boost will drop off to about 10 psi and build back up to 30. Otherwise truck runs great. I checked all the clamps and couldn't find any leaks. Not sure where to look next. Any help would be great. Thanks!