Car overheats out the blue

I have an 02 impala 3.4 and when I
changed my thermostat, 2 days layer
the car overheated on the freeway. I
dont mean gradually even, it shot
straight to the red and made my hot
temp light come on. I pulled over,
released my lever on the radiator cap
and saw I had coolant but the lower
hose was sucked in. 10 min later I
started it back up and the temp went
back to normal. So i drove to
autozone and changed my lower
hose and a week later it did it out the
blue again.  Once again I pulled over,
shut the cat down and started it back
up and it was fine.  I also noticed
something drippin coolant onto my
manifold but couldnt pinpoint it to
the upper hose. I am not a mechanic
but understand somethings. Do
anyone have any ideas??