Higher voltage can mean brighter bulbs - and significant
life expectancy loss. But long before trying to fix things,
first, you should be collecting facts. Locate the problem
before trying to fix anything. That means numbers and a
multimeter. What is the voltage at those 12 volts bulbs?
Those numbers will explain much. Furthermore, what is the
power source providing that 12 VDC? Is it regulated or just a
transformer and some diodes? Finally, what is the AC voltage
going into that power source.

Ike wrote:
We have Malibu landscape lighting and the bulbs burn out after only
about two months of service. They are very bright compared to my
neighbor's Malibu lights and he only replaces bulbs about once a year.
When the landscapers installed our lights they did not use the wiring
that came with the set. Instead, they used their own wiring which is a
much larger gauge resulting in less voltage drop by the time the
current reaches the lights. I am sure they thought they were doing me
a favor, but their generosity appears to have worked against the life
of the bulbs. Is there a simple solution to drop the voltage enough so
these bulbs don't burn so hot? Would a standard light dimmer for 120
VAC work to adjust the 12 VDC voltage?