Coolant gauge fluctuates from normal to hot and back down

While driving my truck (04 Ford Explorer Sport Trac)
yesterday I noticed that the coolant temp. Gauge was
fluctuating between normal and high (about to the red) and
then it'd go back down again. This happened a couple of
times before I got back home and parked it. This is
completely out of the blue, and no prior problems saying
that something was going bad. I noticed that the temp got
higher when I'd sit at a red light or if I got behind someone
in traffic, but would go back down to normal temp. once I
got going again and air got to the engine. Although when I
got it home I did smell something that smelled kind of like
hot plastic or hot dirty water type of smell. Any ideas would
be helpful. Don't know if I should just take it to a mechanic
or try and trouble shoot it myself, so any help or advice
would be gratefully appreciated.