extremely slow when execute select/delete for certain tables only...

I'm facing a very weird problem.
Recently our database run very slow when execute Delete/Select statement
for a few tables only..
The largest table only have 50K rows of data.

When I run the statement from pgAdmin although it is slow but not as slow
as run from webapp.
When I run the statement from webapp, it become extremely slow.
Even a simple delete statement will takes 20-40 minutes to complete.

I already vacuum those tables with full option but it still the same.

What could be the possible causes of this problem?
How can I solve it?

CPU - Intel Xeon 2.40 GHz
Memory - 1.5G
Postgresql version: 7.2.2

Thanks.