IS it a built motor with headwork and cams? My guess at this stage, is:
A .86 ex housing is a little small for an rb30det, and if it has cams etc, that would make it worse
A 44mm gate is a little on the small side if you are trying to run low boost.
Ivve seen this same problem on Todd Wilkes 1400hp Gicottolo, boost was going off the dial with the wastegate valves installed backwards it had twin gt35r .86's(if i remember correctly) twin 50mm wastegates on a built 5.8. The night between qualifying and comp at summernats we pulled the turbos off and machined the ex housings 1.5mm(and wired in an autronic) Fixed the boost control.