Losing Power

Hi All,

I have a 91 Cutlass 2.5 litre 4cyl auto transmission and have a
problem with losing power when cruising up a grade. The car will
normally shift through 2nd and 3rd gear but then start to lose power
when I apply a steady amount of gas to cruise up a low grade hill. I
can stomp it down and kick it back to a passing gear, but if I
steadily apply more gas, it loses power, almost as if it was choking.
Also, when a soon get out of the car it has a eggy smell as if the
wrong gas/air mixture was burned. Has anyone had this problem and if
so is there anything I can do to fix it. On another note my temp gage
will almost reach 260 F before cooling back down as the thermostat
opens up. The funny thing is that it will pulse back and fourth
between 240 and 160 after that, maybe I need a new thermostat.

Thanks for any help.
Smerby