Need help in tuning

My PG server is still going down. After spending the weekend doing a
CLUSTER of my largest table (it's a RAID 1 system with SATA hard disks
and 4 GB memory, mostly devoted to PG) I still have this issue.

When I do a "top" command, 99% of the CPU and about 15% of the memory
is being taken by PG. When I press a "c" in the top UI, I see that
"postmaster" is doing some "CLUSTER". However, I don't do any more
clustering. The only automatic setting I can think of are autovacuum.

So, question: to the degree that my system allows for performance,
what steps can I take to find out what's happening? I see some things
mentioned: I/O, vacuum settings, pg_stats, pg_activity -- is there a
simple guide somewhere which shows me step by step what to do? Google
hasn't been much help. "Postgresql performance tunings" brings a lot
of esoteric articles.

Thanks!