Temporary tables

Hello everybody.
Sorry for my english, it's no my native language. So I have problem with my
PostgreSQL 7.3.3 server. Untill now everything works fine, but now I get
lot's of errors:

Warning: pg_exec() [function.pg-exec]: Query failed: ERROR: cache lookup of
relation 149064743 failed . in table.php on line 169
Warning: pg_exec() [function.pg-exec]: Query failed: ERROR: Relation
"tmp_table1" does not exist . in table.php on line 177
Warning: pg_numrows(): supplied argument is not a valid PostgreSQL result
resource in table.php on line 196
Warning: pg_exec() [function.pg-exec]: Query failed: ERROR: Relation
"tmp_table1" does not exist . in table.php on line 221

The only reason of it as I can imagine is high load to server. Can it be so
and if it's not, can anybody tell me reason of it and are there any options
I can change to avoid this situation in future.
Thanks.