My 91 montero 300 v6 is hesitating while under a load. I put a new o2 sensor in and thought the problem was fixed but 30 minutes later it did it again. This problem occured all of the sudden a few days ago. If you have knowledge please share with me......

 

You need to be more specific. Hesitating like missing? Or it hesitates when you push on the accelerator? Missing is generally something ignition related.