That is not the best answer from the mechanic.
They should know by now with these symptoms you may or may not get any feedback from the computer system.
This is what I would do. I would get a can of starting fluid maybe 2 as they can go fast.
When the problem starts to happen try spraying it into the throttle body. See if the vehicle condition will get any better at all if so we have a fueling issue when hot.
If this does not make any difference the next thing I would do is check spark when problem is present It should be string nice long jump blue ish in color. If the spark is weak this can be caused from bad distrubutor assembly even if it was replaced. I do not think this is the problem but I am throwing it up for you to check for yourself.
Next thing is remove the O2 sensor
from the front. This will rule out a bad catalytic convertor. It allow the exhaust backpressure to be relieved from the hole made by leaving it out. You will get a Engine light but it is totally ok to drive to test the problem this way.
This way it costs you very little to check all of these yourself and not pay the mechanic to keep throwing useless parts at the vehicle.
If this checks out I would suspect a ECM. Engine control module.
You can try putting it into the freezer than drive again when it normally does not run good to see if the cold changes performance.
I hope I have given you some useful information, please kindly leave a positive rating. You can return at anytime to reply but this ensures I will be credited for my time and efforts.