undo header and buffer busy wait

We are using 8.1.7.2 on solaris.
We have usually 20 sessions, and we have 13 rollback segs, each one with 50k initial, with optimal set to 500k. Even though, we still get  a lot of undo header wait and the buffer busy wait is always on rollback tablespace, how should I do to improve it?

Our I/O wait from top command shows we have constantly 5 to 10%

Thanks  

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