So could be running lean if it begins after the warm up period when running rich and with gentle pressing of the gas pedal after! My mechanic wasn't convinced that it was the O2 sensor on my 4R especially being the downstream one, he said that has no relevance, but after my own research I was sure it must have, why would it be there ? Pretty sure it was giving false reading to the ECU and screwing up the mixture, the OBD11 reader, ($19 Thanks Amazon) kinda confirmed this too. I picked up a sensor from NAPA BTW.
Are all the hoses snug ? there could be an air gap/leak somewhere, something to check.
Here are the rust areas I have right now. Not too bad for 18 years eh ! There's a couple of winter snow shots too.