2001 DTS Need a little help here

Hey guys/girls, so first things first im new here so my problem MIGHT have been solved already, it would be great to get the answer sooon though

Basicly, my problem is this. I have a 2001 DTS it started out running great but then it started over heating (needle passes center mark) and the car would start smoking..smelling alot like burning oil/coolant.
Got that fixed after changing the thermal stat and it started running normal again
However, the car still smokes, the smoke smells somewhat like coolant and the coolant level IS infact going down. Along with the coolant going down the oil slowly does too. :hmm:
And NOW the check engine light came on, as the car stops too, you can feel a slight misfire so though came in..we changed all 8 spark plugs with brand new bosh platnum 4's..yet we can still feel misfire and check engine light is still on :helpless:

Some Questions!!
First of all, is it normal for t he car to run with the temp. Right in the center?
Secondly, does anyone have ANY idea whats going on with my car and how i can fix, or reduce this problem?

THANKS , i know its alot to read but it will help me TONS