1998 Dodge Dakota Feels like a bucking in the engine

I just finished replacing my head gaskets, exhaust manifold gaskets, intake gaskets, spark plugs, and exhaust from the y-pipe back. when reinstalling components i didn't put the factory air box back on and installed a cone filter from a local parts store. I'm gettin a Check engine lite for O2 sensor high voltage, and when i have the throttle at less than 40%, under load at low RPMS, the engine starts loosing power, then popping and bucking, and then when I go WOT it's accelerates once again. Please help.