Car dies and says voltage and now misfires

I have a 1997 Chevy lumina.
The last few weeks my car was running
great until a week ago.  My car then started
to want to die while on the road and the
headlights would dim and the heater fan
would get really weak. Then 2 days after
driving it like that it just died on the road and
said voltage on the dash. I barely got it to
start back up and get home. I then got a
mechanic to come and say it was the
alternator so he replaced it with a new one.
As soon as we started the car though the
check engine soon light had come on for the
first time and was blinking. The car has been
running extremely sluggish and still wants to
die when idling or changing gears or hitting
my breaks. What do I do now? I am so lost
and confused.