2003 Hyundai Elantra GT engine light warning and car has acceleration problems

This is my first car and I am having some problems with it. It is a used Hyundai Elantra GT with 79000 miles on it. I have only taken it on 2 drives over 40 miles and each time around that distance it starts sputtering and losing power. After pulling over to the side of the road and waiting 20 minutes or so each time, the car started up and drove fine. The first time the check engine light did not come on, but the second time it did. We went to Shucks and did a engine diagnostic test and the codes P0705 ( Transmission range sensor circuit malfunction - PRNDL input - ) and P1166 ( Heated oxygen sensor 1, bank 1, at control limit ) came up. What can I do myself to find out and fix this problem before going to a mechanic? I am new to cars but pretty handy so I hope I can do something myself first. Any help will be appreciated. Eazy

I'll bet your head gasket is leaking antifreeze into the cylinders, it's fryed the catalytic converter to the point of it collapsing, thus your exhaust is clogged. This leads to overheating and loss of power, which goes away once you stop and cool off. Is it losing antifreeze? Is the exhaust pipe "cool" after the catalytic converter?

I have a 2001 elantra GT 120k miles.
The cel came on and I read the code P1166
It only had that one error, so I looked up what it might be.
It turns out that this is a very vague error code, it could be
bad O2 sensor #1 ( the one on your exaust manifold)
a leak in you exhaust manifold / intake manifold
bad fuel filter (it's in your gas tank with the fuel pump)
bad fuel pump
bad gas
plus a few more.

This error drove me nuts.
It turned out to be the MAP sensor.

One clue that might be helpful to zero in on the map sensor is when you start the car when it is cold , as usual it's idle speed is high and you don't notice any problem.
But if you start the car when it is already warmed up it will idle very rough for about 15 seconds then it will smooth out to it's normal idle.
once I changed the MAP sensor the 15 second rough idle after warm up went away.

Note: The MAP sensor is located about 12" to the right of the oil fill cap. I bought mine on ebay.