2002 Bonneville dies

Hi group...here's my tale of woe. Wife's car is dying quite often. It's the

3.8L SE with about 61K miles on it. Things replaced or cleaned to date: new fuel pump, new fuel filter, MAF opened and cleaned with MAF sensor cleaner, IAC taken out and cleaned (both had very bad black sooty deposits on them), new air filter and spark plugs 1,2,3,6 replaced (Ican't find a way to get to plugs 4,5). Removed plugs look normal for 61K miles. Scan code gives up P0172 (system too rich bank 1). Problem not too bad when engine is cold but gets worse when engine reaches temp. Also when the engine is at temp, the idle will start to oscillate to the point of engine quit. That's all I can think of at the moment. Any clues I'm missing? Thanks...