Create interval using column value?

Hi

I have a table (A) with a integer column called build_interval, which
contains the number of milliseconds betweeen builds. I have another table
(B) with a timestamp with timezone column called built_on. The two tables
share a column called join_col. What I want to do is find all the rows in
A which were built before now() - build_interval seconds ago. If I could
just write this in SQL it would look something like:

SELECT A.id FROM A JOIN B ON (join_col) WHERE built_on < now() - interval
'build_interval seconds'; This does not work at all - I cannot find a way
to build an interval using a column - casting an integer as an interval is
forbidden. Can anyone suggest a way to achieve my desired result? I'm not
wedded to the interval approach. I can probably change the column type of
build_interval if necessary.

I am using postgres 7.3.4

Thanks in advance for any help.