1999 Cadillac STS Fluctuating Temp Gauge correlates to high

My car was running good until I was hit with multiple problems simultaneously. The car run perfect, but after the engine warms up, the needle begins to rapidly drop saying the engine is ice cold , the jumps back up to the normal temp. Eventually, the needle will stay on "C" and other problems arise. When the gauge reads cold, the car idles between 1500 and 2500 rpm (the car is constantly trying to pull when you stop). Then when I apply the gas the engine chokes and cuts off. When I try to start the car, the engine turns, but won't start and smoke comes out of the exhuast. After a while, the car will run normal, but this problem happens somewhat randomly. I would really appreciate if i could get an answer, because I want to find out if it's worth the fix or if I'm better of junking it.