Mitsubishi Galant 1999 radiator issue

Ok, so my car has this problem thats been
happening for a long time now but has started to
get worst. My car rattles and shakes quite a bit,
which might be the cause of it. BUt here goes the
detail: I'll be driving to work and then i'll come to a
stop light or a stop sign and i'll slow down to stop,
then my car will stop going and my caution lights
will turn on and my radio will still be on, but my
power steering will turn off (barely allowing me to
steer out of the way as to not block the road) and
to reset it i have to turn my car off and back on
again. I used to think that the problem might be my
coolant because my car doesnt pull coolant from
my storage so i have to pour coolant into the
radiator directly, like i said, this is what i thought
was the problem. If anybody can troubleshoot this
i would be very grateful