OK.. before I can even begin to help further, I'm going to need to know what codes are stored in the computer.
As for the O2 sensors.. I would unplug them and double check the terminals at the connector.. make sure none are pushed back.. or bent.. or corroded.. etc. Also, if you've been splicing in 'universal' sensors, then you need to make sure you're splicing into the correct wires, and that you are SOLDERING them together and using shrink wrap to properly seal the splices so they don't get contaminated with moisture.
You also need to physically inspect as much of the wiring as you can, to make sure none of it is laying against the exhaust somewhere and melting... or coming apart.. or whatever.
O2 sensors can also get contaminated and fail if the engine is allowed to run rich. It's possible you have another problem going on (such as a ruptured fuel pressure regulator.. injector sticking.. low compression..ignition problem, etc) that is allowing unburned fuel into the exhaust, and after awhile the O2 sensors get so saturated with it that they can no longer function correctly. They will usually clean themselves up once the engine stops running rich... but it's still possible that something else entirely is causing your O2 sensors to fail.
You mentioned that even with it's 'fixed' it still doesn't seem to have full power... that sounds like it's not running on all 8 cylinders... I would suggest that you chase the misfire before continuing.
You can also try removing the O2 sensors and drive it around the block. Yes it will be VERY LOUD but if you notice that the power has been restored, then there's a good chance that the catalytic converters
are all plugged up (and you have power now because the holes opened with the O2 sensors being out allow it to breathe).
So you see there are still many possibilities here.. I feel the O2 issues is a result... not a cause.. of your problems.