Surge around 5000rpms?

I've got a 92' se-r, and for a while now I've noticed that when my car accelerates it picks up (like vtec) at 5000rpms? It always does it at 5000rpms. Why wouldn't my power delivery be smooth all through the rev range.

Another thing is I've been getting 18mpg lately. I just did a tune-up so I'm thinking o2 sensor maybe?

Thanks for the help.

id almost bet 02 sensor will fix both problems...check back with us after you do it