the above from a converstaion I had re same problem on mkiv.com from john cribbIf any of these valves aren't working properly, you'll have low end boost problems:
1. BOV (boost will get bled back to intake)
2. EBV (loss of exhaust energy)
3. EGCV (loss of exhaust energy)
4. WG (loss of exhaust energy)
Now if the EGCV, or EBV aren't working properly, you should have strange transition problems, as well as low boost on the low end.
If the problem is the WG, you should suffer low boost consistently, whether it's only #1 or 1+2.
If you're suspecting a stiff/sticking EBV, try pressurising its actuator with air and see if the motion is jerky. This is a real tough one to watch, especially on a hot engine.
Being a lazy individual and knowing that Supra failures tend to follow patterns, I'd look hard at the EGCV actuator. Mohd and others have found this to be one of the culprits of low boost.