ORA 1562 Rollback segment problem

Hi,

	I have defined the table space for Rollback segments to a size
	of 4M. Our Product Test folks running a particularly large transaction,
	got ORA 1562. The transaction sometimes works and sometimes doesn't.
	In looking at the four segment sizes:
		RB1 532K
		RB2 665K
		RB3 2.5M
		RB4 400K
	Can anyone have a clue as to why the transaction sometimes fails?
	4M seems like it should be enough space. Given that I'm in the
	situation, what is the best way now to get out?

	HP-UX with Oracle 7.0.16.4

	Thanks.