My neighbors 1998 Bonneville dies when it is warmed up

I have been trying to help out a neighbor by taking a shot at her Bonneville warm up death problem. It dies right after 10 minutes of run time when it is cold. It loads up real rich and wont stay running unless I floor it. When I let it cool down for an hour or two itr runs fine for 10 minutes again. I did the old code reader check and everything passes?!?!. I am leaning on a O2 sensor but which one?? It has 4. Any ideas of what else it might be? It is a 1998 Bonneville with a 3.8. OBD 2 let me down.....

Welcome to AF, first check for fuel in the press reg hose, indicating a bad fp, second, re-scan the car and make sure the coolant temp sensor indicates getting warm as the engine warms up, not stuck at say -10 degrees causing it to be too rich once running. This will require a scanner not just a code reader.