Yesterday coming home from work in -10 C, a 26 km drive,

I was getting just cold air through the heater. I was watching the temp

guage all the way home and as I got near my home about 20 minutes in,
the temp guage on my pickup was at the red line.

Once parked in driveway, I noticed the rad pushingbubbling fluid into
the
overflow and could smell hot antifreeze.

I have noticed the past few weeks that the temp guage has been rising
but then would suddenly start to go down esp. if I turned on the heater
and it seemed unusual at the time
so I have been watching it. --yesterday was the first time, it did not
go down. I went for a little drive later that evening and this time,
the temp rose even faster -7 minute drive to move temp guage close to
red and
still no heat from heater.

I assumed/read that you would have one or the other,
that is, engine overheats so thermostat is stuck in closed position
No heat in car so thermostat is stuck in open position.

I seem to have both scenerio's going on.

Any thoughts -is it even the thermostat?
if so, I know the thermostat is relatively inexpensive? should I buy
3rd party or from dealer
and finally, how long/shop time to replace one (I am assuming one hour
or so) and is it worth trying to do it myself?

many thanks in advance