what could be the problem to my 1991 mr2 turbo?

Black smoke while doing this or not? If yes, probably a leak between the intake and the throttlebody. Check all vacuum lines. The way you describe it, it seem to be a clogged open wastegate (the are internal inside the turbo). Could be aslo a stuck open blow-off valve if it's cheap or a bad MAF (Manifold Air Flow) MAF are getting stuck sometimes. If you're engine Speed cut when the boost comes on it's probably because you have a bad/faulty boost pressure sensor. The 3SGTE is set to hit a fuel cut at around 12psi. So if you installed a manual boost controler without a fuel cut defencer to trick the fuel management system, it will cause your engine to kinda Stop to protect it and you'll regain power when safe boost pressure is reached. You'll get a check engine for that, Code 35 I beleive if I remember.