my oxygen sensor problems...need help

[karnoff]

my car a 1990 dodge spirit with the mitsu 3.0L V6 has had some major problems recently. about 3 months ago i got a new tranny, head gaskets, cleaned injectors and more, and my dad says the mechanic put a new O2 sensor in it as well. well just about two weeks ago, i started getting bad mileage, so i took it to the shop checked it and they said bad O2 sensor, so me and my dad got a new one put it in and it worked fine until i filled up on my next tank of gas, today, and i began getting really bad mileage again, so im guessing its the oxygen sensor, but two O2 sensors in a week? does anyone know what could be causing this mishap?

[Polygon]

It could be the fuel you're using.

[karnoff]

well i just started getting the bad mileage as soon as i pulled out of the gas station. and it cant be the fuel because there is still fuel from before in the line, so i just told my dad and he said to just disconnect it and see what happens. this mitsu 3.0L V6 is quite possibly the worst engine i have ever had in terms of problems...its quick and all, but problematic.