Slow performance with no apparent reason

I am in the process of  adding PostgreSQL support for an application, in addition to Oracle and MS SQL.
I am using PostgreSQL version 7.3.2, Red Hat 9.0 on Intel Pentium III board.

I have a query that generally looks like this:

SELECT t1.col1, t2.col1 FROM t1, t2 WHERE t1.x=t2.y AND t2.p='string' AND t2.q=1

This query is strikingly slow (about 100 sec when both t1 and t2 has about 1,200 records, compare with less than 4 sec with MS SQL and Oracle)

The strange thing is that if I remove one of the last 2 conditions (doesn't matter which one), I get the same performance like with the other databases.
Since in this particular case both conditions ( t2.p='string', t2.q=1) are not required, I can't understand why having both turns the query so slow.
A query on table t2 alone is fast with or without the 2 conditions.

I tired several alternatives, this one works pretty well:

SELECT t1.col1, t2.col1 FROM t1, t2 WHERE t1.x=t2.y AND
     EXISTS (
             SELECT * FROM t2 t2a WHERE t2a.p='string' AND t2a.q=1 AND t2a.y=t2.y )

Since the first query is simpler than the second, it seems to me like a bug.

Please advise

Yonatan