From the descriptions you have given, it sounds like Lamp failure on both fixtures.
HPS Lamps will cycle on/off when they near the end of their "Life" - more frequently as age increases (age = operation hours).
These must be 150 Watt (or less) HPS Lamps with Medium bases - which would be appropriate from the 54 Volts measured at the Lamp Holder (Med. base socket). These Lamps are 55 Volt Lamps.
If the Lamps are Mogul Base, then the open circuit Voltage is kind of low, and a Ballast replacement may be required.
Be sure the Ignitor on the newer fixture is operational, by installing a known-working HPS Lamp of the same Wattage in the fixture and powering it up. If Lamp does not start, suspect Ignitor failure.
If you have an equivalent, known-working Ignitor, swap it out with the existing Ignitor in the fixture being tested, then power it up to verify Lamp starts.
One last thing - or two
[*] Verify correct wiring between Lamp, Ballast and Ignitor,
[*] If a PF Capacitor is used, verify it is connected on the "Input" side of the Reactor coil... where the 120 VAC circuit connects,
[*] Verify the Reactor's leads are connected to the correct "sides" - e.g. Input side to branch circuit "hot" wire.
If you need wiring Schematic assistance, check the Technical Reference section of ECN for H.I.D. Ballast Schematics.
Let us know of the outcome - and most of all... Good Luck!
edited to fix a blunderful statement!
[This message has been edited by Scott35 (edited 03-15-2005).]