2000 Buick 3.1 Rough Engine

I'm at my wits end!
I've spent about $800 trying to fix this problem...rough engine operation at higher speeds under load (40+ mph). Seems to smooth out
at full power levels. And it is intermittent to boot!
At first, it feels like an ignition problem. But scopes show nothing. No OBD II codes either. One mechanic thought it was wheel bearings (I was desperate and he was the "expert"). He replaced one, and the problem whnt away--for a while. Then it came right back.
I'm thinking of purchasing an OBD II analyzer and watching the engine parameters under load. But this is antother $160 and I'm not sure I will be able to detect anything (being an amateur).
My gut feeling? A possible bad injector dumping inappropriate fuel into one cylinder.
Any thoughts from anyone?
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