Persistent engine codes 1996 Buick Century

[Dankyogi]

I have been fighting a three year battle with the Service Engine Light on my 1996 Buick Century with the 3100 L82 (M code) engine. The codes are P0171 (Fuel trim bank one condition) and P0401 (EGR system fault). It is the same two codes each time, and they always occur together, never separately.
These two codes pop up every few months. I've replaced all three coil packs (twice), wires (twice), plugs (twice) and EGR valve (twice). Following the repairs, the SES light goes away. But without fail the same two codes will rear their heads again within months of the parts being replaced.
The car is fairly high mileage (165,000), and is driven daily for work and errands. The symptoms the car exhibits once the codes register are: rough idle on cold start, random misfiring, slight loss of power, random hesitation during acceleration and rich-smelling exhaust.
I've also replaced the intake manifold gasket recently, though for an unrelated issue (coolant leaked into oil and outside engine).
Does anyone have any insight to this issue, or should I just expect to spend money on this problem every year?

 

[MelsRegal]

For the P0171 all your repairs appear to be ignition related while the code is a fuel mixture problem. Have you read this link concerning the causes for P0171 and the solutions?
http://www.obd-codes.com/p0171

 

[Dankyogi]

For the P0171 all your repairs appear to be ignition related while the code is a fuel mixture problem. Have you read this link concerning the causes for P0171 and the solutions?
http://www.obd-codes.com/p0171

I've checked for vacuum leaks and the manifold gasket is new. I am replacing the PCV today and cleaning the MAF. The fuel filter is new, and fuel pressure is fine. The only things from that list I haven't done is replace the O2 sensor, diagnostics that require specialized equipment and inspecting the injectors.
Could the O2 sensor have been at fault the whole time, only sending fault codes randomly? The code disappears for months indicating no issues, then appears to stay until repairs are made.
Is there a common cause that would activate both the fuel trim code on bank one and an EGR fault. The two codes ALWAYS activate together, never on their own. The EGR fault always returns after replacing the EGR, and the fuel trim code on bank one appears with it.