DTC P0171 on 1998 Oldsmobile Intrigue

My 3.8L V6 with 75K miles has for the last 4 years had a very intermitant DTC always P0171 usually correcting itself. After several trips to the shop (always at a very rep Cadillac dealer) and they cannot reproduce or find any problem. Now the problem is constant. I have torn the engine down to the heads. replaced intake manifold due to melting caused by the EGR stove pipe. replaced MAP, MAF and both O2 sensors. I have replaced fuel filter. I have had all the injectors cleaned and tested they are all flowing evenly and at the top of spec. range. I have tested for any vacuum leaks. The car runs like new, with no discernably hesitation or other drivability issues. Yet the DTC P0171 still resets, I cam clear the code and drive for a while then the code resets. There does not seem to be any difference how I drive, easy, hard, fast, slow, idle. the code just reappears. Any Ideas would be appreciated.

Asked by for the 1998 Oldsmobile Intrigue
In the past I have seen vacuum leaks from the EGR tube, which I think you are referring to as the "stove pipe". The tube can develop small cracks or not seat correctly at either end. I would recommend to recheck the EGR tube, as engine vacuum is present all the way to the EGR valve.
entire intake system and all vacuum accessories have been checked for leaks, with none found. also the "stove pipe" on this engine is the small tube which is pressed into the lower intake manifold and passes through the upper intake manifold to the intake air stream.
I was referring to the EGR outlet tube which attaches the to the bottom the the EGR mounting plate and the rear edge of the lower intake manifold. This tube can crack or not seat correctly at the intake manifold or EGR mounting plate.