And your board settings? Input should be a tick under 200psi, and i put dwell on 34-35, BIP at 6 for normal warm weather, but have wacked it way up to 25 for really cold weather. If you still get only 500 shots from a tank, could be you have a small leak somewhere, including it could be your air system with a leak.
something else to note is even 10% wear on an oring can cause issues, its not like they are all good till they just snap one day. Regularly (like 3 times a year if you play most months) change the orings on moving parts.