18G Choppy Boost in 5th Gear

[low_&_loud]

Hey guys -
I've had an 18g with supporting mods (below) on my car for about 4 months now, originally with a grimmspeed EBCS, but it was creating wavy boost (overshooting then undershooting target boost and oscillating a bit up top), and I was having some boost creep pretty bad in 4th/5th. I went to a grimmspeed MBC per my tuners recommendation and really love the slightly quicker spool/ rock solid boost, except now especially with it getting cooler out, when I roll into the throttle in 5th, the boost comes on then starts to get choppy. It is a quick ch-ch-ch-ch-ch sound, but it is under throttle which from my understanding is not compressor surge, please correct me if Im wrong.

The weird part is sometimes it gets to the point the car feels like its bucking a bit but I always let off the gas as soon as I start hearing this, Ive only figured that out by staying on the gas long enough to see if it goes away and almost never does once it starts (unless I pedal it a bit). This leads me to believe it is some kind of surge, but thats where Im confused. Ive read the FAQ and understand what WG flutter is but this seems kind of extreme. It doesnt do it if I shift into 5th and Im already ripping on it, and it doesnt do it in any other gear... Only if I roll into most or full throttle from a relatively low RPM (as in passing someone on the highway) does it happen.

The tuner's suggestions was to tighten the wastegate actuator arm, which I am going to try, but I wanted to get a few more opinions here before I start to play around with it...Any ideas on whats going on here/how to fix it?

Thanks!

Relevent Mods:
18G
Grimmspeed MBC
STi Uppipe
Cobb Catted DP, Catback
Perrin Inlet
DW 750 Injectors
TGVs deleted
STi TMIC
Walbro 255
NGK BR6 plugs
KSTech Short Ram Intake