failure to extend rollback segment 20 because of 1562 condition

Hi Friends,
Some days I get the following errors in my alert log:
FULL status of rollback segment 20 set.
failure to extend rollback segment 20 because of 1237 condition.
failure to extend rollback segment 20 because of 1562 condition.

After about 2 hours, I get the message saying that FULL status of rollback segment 20 is cleared and from then on there are no more messages .

My database is having two instances(OPS) and I have 6 rbs for each instance.Their sizing details are:
INITIAL 80M NEXT 80M MINEXTENTS 2 OPTIMAL 160M

How can I avoid the 1562/1237 errors? And how can I clear the FULL status myself instead of letting Oracle do in its own time?
All help is appreciated.
Thanks
manjunath