89 525i hesitation problems

[dmc_2005]

while running at all different speeds i suddenly get a severe hesitation. check engine light may or may not come on. when it has come the code is for o2 sensor. engine has 195k but have just rebuilt upper 1/2. have tried new crank position sensor, but cannot duplicate problem to ensure proper diagnosis. air intake boot had cracks and that also was replaced. when hesitation does occur rmp,s drop to nothing, but engine does not die just lurches upon the hesitation.