The BOV's have a screw for adjusting, this works like a tensioner.
When the screw is wound one direction, say in, then the spring inside becomes harder, therefore needing more pressure to open, so then it will open only on high rev's.
When wound out, the spring is softer, then it will open on low revs, which means less pressure to open a softer spring.
Just find the BOV, and screw the adjustment screw out a few turns, you should be fine.
If you want it to open on low revs = undo the screw
If you want it to open on high revs = tigthen the screw
If you open it to much you may get stallng issues when rev's drop, and I think thats also the case with it being too tight maybe?
Your getting the flutter because the BOV is set to firm or harder, =high rev's. Its not opening on low rev's so your getting flutter.
Found this on a site which talks about how to set your bov:
Hope that helps.
Thanks,
Abu