severe cutting out.

so i bought this 87 supra, which has basically a rigged up turbo. its a ct26 but it was sent back and had a different impeller and was milled out. so 1) im driving and in 1st gear it revs just fine up to 6k. every gear after that it cuts out after 3k-3.5k and to regain acceleration i have to feather the gas pedal. 2) i was driving yesterday and after putting about 50 miles down, lost all power and cant rev past 2k. it sputters and bogs and blows black smoke. i noticed the downpipe had shimmied away from the turbo so i figured the o2 sensor was tripping out. i re-tightened the bolts and tried again and it did the same thing. little help anyone?