Toyota Repair: 95 sr5 4runner, engine codes., temp guage, headgaskets

Question
I have a 95 4runner V6 4x4 that's been giving several engine codes latley. The first code it gave was a 71 code but after finding some disconnected vacumm lines and reconnecting them has gone away. Next, it gave a 14 and a 26 code and I was told that it was probably the 02 sensor so I replaced the one before the cat. The code went away for a while then came up with a 25 code and is running hot. I replaced the thermostat and the engine code went off, which was suprising because I dodn't think they were related. The truck got hot again after replacing the thermostat after driving 6 miles. I stopped let it cool off for only 10 min and started going because the temp guage stayed below the half mark. The truck has a smell of gas which is less since the O2 sensor was replaced and also has been difficult to start cranking for awhile before turning over. Supposedly the previous owner had the injectors looked and, 1 rebiult and had them resealed. Any ideas of what could be going on ? Everytime I think I have it nailed down somethig else comes up.  

Answer
The O2 sensor codes are coming on because there is an engine problem, replacing the sensor does not help. It's not the cause.
These engines had a problem with the headgaskets so I recommend that before doing anything else that you have it checked for that possibility especially since the engine has a tendency to overheat.