Q: Engine rattling at high rpm, P0180 code stored, unsteady rpm at idle, no vacuum leak, mail carrier vehicle, please help!

This car is old and yes, it lives a stressful life because I deliver mail in it, stopping and accelerating at over 650 mailboxes a day. That said I am storing a P0180 code (if I recall correctly) but it was an O2 Sensor - Bank 2 System too lean at idle. This code was accompanied by unsteady RPM at idle, it actually stalled once or twice at a mailbox, and a RAPID RPM drop as soon as I let off the accelerator. All of a sudden it fixed itself for about two weeks, now it's doing it again but with additional nuisances. It started out being vibration of the car (especially if AC is on). Then I developed pinging in the engine when the car reaches higher RPM and especially when under load. This car is my livelihood and I desperately need an answer. I have seen the inside of my motor and it is clean as a whistle for 232,000 miles. One mechanic told me all of this was a Fuel Temperature Sensor but there's no such thing on my car!! Please help!

My car has 232000 miles.
My car has an automatic transmission.