Read up on www.mkiv.com about the FFCD mod and also the HKS FCD. Also double check for the BCC, most guys will mount it under the passenger side kick plate. Pull up the carpet, undo the 2 10mm bolts at the bottom of the black box/kick plate and remove it. Now you will be able to see the car's ECU. Look for a little black box piggy backing on it with some wires spliced into the ECU. That little black box is most likely a BCC.
Stock Supras normally (in my experience) boost cut at 14 psi or so. Usually if you hit 14psi for a few seconds in a row it will boost cut. So look into things further. If you have the FFCD mod quickly lose it and switch to a BCC.
Why the previous owner put in a Electronic boost controller and *supposedly* never did a boost cut mod is beyond me. Doesn't seem to make sense.