One. Decrease your log buffer Say to 1mb (I dont know your current size but 1 m is normally good).

Two. You might be keeping other datafiles or temp files in the redo log area. Move redo to a separate device. Do not keep them in RAID 5.

Three. See the log file size, if frequent switches are there increase the size.

I also face frequent log switches in one of my db with solaris 8,
might be the undo the reason for the redo fast filling.