2009 Toyota Camry LS 4-cyl.

My Toyota Camry developed a maddening problem well over a year ago that
has me to the point of changing cars, and, at age 84, I desperately do not
want to do that. It began starting,,,then immediately stalling, then starting,
and stalling again., and this problem continues...and continues. The "Check
Engine" light is on when this happens. You have to "wait it out" when this
problem manifests; the longest I have had to wait for it to start  and KEEP
running is almost a half-hour. The waste of time is considerable and when it's
dark and freezing out it's more than an inconvenience. I've used the best
mechanic there is, for some 25 years, and when HE's stumped you know
you're in trouble. And he's stumped. I've checked every lead and even wrote
to Toyota USA, Engineering, Plano, Texas. I rec'd a lengthy letter in
response, which was of no help at all. A gearhead passerby said to pour a
can of fuel injector cleaner in, fill the tank up with Hi-Grade gas, and run it
'hard' for a while. At first I thought this was "it," then the problem reasserted
itself and I was back at Square One. When the computer codes were read, it
said Oxy. Sensors One and Two. My mechanic and his understudy worked a
long time on this, involving undercar welding, and thought that would take
care of it. It didn't. Some think it's the fuel filter in the gas tank but that's a big
project, plus most think it's "very low likelihood." I'm at the end of my rope on
this and am at the brink of changing cars, which I'm in no position to do. The
car now has 126,000 mi. on it. I had a previous (2005) Camry with no such
problem. Help if you can.