Strange behaviour under heavy load

Hello.

I have found that sometimes heavy loaded PostgreSQL begins to run all the
queries slower than usual, sometimes - 5 and more times slower. I cannot
reprocude that, but symptoms are the following: queries work very fast for
5-10 minutes, and after that - significant slowdown (every query, even a
simple one, works 5-10 and ever more times slower), disk write activity
grows too (but not everytime - I don't know exactli if there is a direct
correlation). 2-3 seconds, then - performance restores back to normal.

Autovacuum is turned off.

Machine has 2 processors, huge memory, fast SCSI disks.

I understand that there is too less information. Please advice what to
monitor for better problem discovering.