Gas engines heat the engine's intake air during the warm-up period - a
small duct routes the intake air to a heat source like the cylinder head
or exhaust etc. I have the suspicion that this "system" isn't working
correctly on your motor.

As the motor warms up this device normally scales back and the computer,
anticipating warm air to be available leans the air / fuel ratio. So
when the warm air is missing the mixture gets too lean and the motor stalls.

If I'm correct, this should be a simple fix.

If your mechanic have proper tool to electronically view all components,
then he would have figured it out. But I would suspect the OVP relay
first... Overload Protection Relay.

Thanks for all your kind suggestions. My mechanic also replaced the
overload protection relay, all 6 fuel injectors and seals. The car now
runs like new but every other time it stalls at the first stop sign.
When I start it up again, heat blasts out of the heating vents, which
it did not do before it stalled. Do you think the problem is the O2
sensor