index error :- index ------ not a btree

hi,
i am facing the above quoted error " index ---- not a btree".
Scenario here is
1) installation of Postgres7.3
2) creation of database using initdb
3) creation and population of some tables using a sql script
4) reboot

after reboot when i do
vacuumdb -f -z -U postgres -d postgres (previously created database)
vacuumdb -f -z -U postgres -d ispcab (previously created database)
vacuumdb for ispcab gives error pg_toast_??? not a btree.

i have scanned through previous posts to find out that this is due to reboot occuring without database server properly shut down and on restart ...indexes get corrupted.

but here the situation is a bit diffrent
if i put a delay of around 120s between the two vacuum commands, everything works fine.
Can anyone help me on that. its urgent

Thanks in advance
Yogvinder Singh