Car dying while driving

Hi, sorry, don't know what the trim is sol just
selected ce to select something. When I purchased
my Toyota 1.6 second hand and it was idling in a
carpark when a person mentioned that I can't idle
this particular year. I thanked him and didn't know
why this would happen. Anyway months later the
battery warning light would blink on and off and
start to get worse. It wasn't until I was at a drive
through that took ages in the line the battery light
started blinking again, after a bit of time waiting in
line the radio turned off and as I grabbed the food
the car stopped and wouldn't start again so had to
be pushed into a car park out of the way. Was able
to get a jumpstart and go home with no problem.
This has happened 3 times since then depending
where I have to drive. Where there are more traffic
lights the problem starts again it seems. If the
battery warning light comes on I start taking the
back roads just in case car stops again. Radio
turns off and the rpm indicator starts wriggling
sporadically. We replaced the alternator belt as
that looked terrible anyway but made no
difference. Swapped batteries from vehicles, that
didn't help either. Could it be the alternator? Need
help. Thanks.