Surging/rhythmic bucking at 30-45%throttle

[l-eater]

I am working on a turboed 2003 GMC and I am trying to figure out a surge/bucking problem which has me kinda stumped. The truck runs ok at most throttle positions but when slowly increasing throttle from about 28% throttle up to about 45% there is a very pronounced bucking which shows on the RPM log as similar to saw blade teeth. I cant see anything on the log which would cause this, am I overlooking something simple??

[l-eater]

TTT, anybody have any ideas??