It sounds like it is some other issue not volts. Earth loop? noise?
If you want to obtain a constant output voltage, use a DC DC up down buck converter like this one
http://tinyurl.com/mayvd8a
This up down converter will transform down if the input is over 12.7 and up if it is under 12.7 (or any voltage) You can set a wide range of output voltage from a wide range of input voltage. Note current is limited to around 3A.
Input Range：3.8V ~ 32V
Output Range：1.25V ~ 35V
Input Current：3A ( max )