What year is the car?
99 svt's and older used a regulator. The pressure should stay around 40 when the car is off.
If the pressure is dropping then you could have a bad regulator, or (rare) stuck-open injector.
Definitely something to look at.

Tom

Former owner of '99 CSVT - Silver #222/2760 356/334 wHP/TQ at 10psi on pump gas! See My Mods '05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red '06 Mazda5 Touring, 5spd,MTX, Black