Are you using a manual boost controller?
If so i'd say check that first, remove it and run a straight vacuum hose to the wastegate. I had the same sounding problem... turned out that the MBC was not working correctly and caused the wastegate to stick open (thus the fluttering sound and lack of power since its dumping exhaust gases). If you don't have a MBC i'd say check the wastegate directly first.
2008 Toyota Rav4 V6 AWD Sport - Daily Driver -
www.SpoolnU.com - ALPHA. SIGMA. BOOST.