Well I got up with a game plan today to go deep into this problem and hopefully fix it. I changed the MAF Sensor, (again) TPS, COIL, IACV, ECM, fuel pressure regulator, secondary TPS, hoses, wirings, etc, etc, etc. I did all the possible fault isolation I could think of and nothing did the trick. After tricking the fuel system I changed the fuel pump and the car is back in business. So now the car is working fine again but I have all these LS400 parts in my garage, all of which works fine but none of which can be returned. I may open a used parts store until I can get rid of them. Thanks for the assistance all, CuriousB you devotion to helping others here is note worthy and I appreciate you time. I have plenty of troubleshooting steps and maintenance for the 91 LS400 in pdf file format if anyone need some information.
I am glad you finally found the problem. Why not attach the PDF in your next posting?