Chevy Express van issues.

i have a 2006 chevy express 2500. Its having some issues. 1st, its randomly
hard-starting. after cranking it for 4 or 5 seconds, itll run like crap until i
feather the gas pedal. then when i rev the engine. theres a puff of dark
smoke.

second issue is when im driving it, regardless of speed, the temp gauge will
drop all the way to cool and the ride gets really jerky and unsmooth.
sometimes the guage will go back to normal on its own and the van will run
fine. other times, i have to put it neutral and rev the engine to get the temp to
pop back up to normal.

anybody have any ideas as to what this might be?

thanks in advance

-charlie