Olds Intrigue 3.8l run problem bucking issue?

I have a 98 Intrigue with a 3.8l in it, runs great most of the time, but randomly at speeds of 40 or less has an intermittant buck or stumble to it, only stalled once so far. It finally threw a ck engine light on with a oxy sensor code, I put a new 1 in and light stays off, but the damn bucking is still there. I cleaned the throttle body and gave it a sea-foam treatment, and some dry gas, with no better results. Any ideas? thanx!