I'd do the leak check first. That car is now 12 years old and those
vacuum hoses have had to endure 12 years of heat, etc. I would not be
the least suprised if one of them is brittle and leaking (replaced the
hose from the intake manifold connection to the plastic hose to the
HVAC system on my '94 Regal just recently).

The manifold vacuum source is a multi-port connector on top of the
intake manifold just behind the throttle body - you'll find a group of
hoses connected there, including one to the brake booster (that, one
is the largest), one  to the transaxle, one for the HVAC system.and a
couple of others (cruise control and evap. system).

Regards,
 Bill Bowen
 Sacramento, CA