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This is Ms Wrench and I can help you with your 1990 Oldsmobile Regency 3.8L.
From what you are describing it sounds to me like the problem is with your fuel pump. If you are getting spikes and drops in fuel pressure that coincide with the engine missing then the problem is most likely erratic operation of your fuel pump.
One thing you can check is the fuel pressure regulator vacuum line to see if there is any fuel in it. If so the regulator should be replaced, but the problem is likely to be a bad fuel pump. Especially since you are having the running problems you are with no codes setting.
I don't know if you would have the equipment, or maybe your mechanic does, but a low amp probe of the fuel pump circuit on a graphing meter would tell you immediately the condition of the fuel pump. If you want more info on this let me know.
With all the other repairs you have done I am advising you that you probably need a fuel pump at this point.
Let me know what you find out,
Thanks for using JA