It depends on the length of your voltage variances.
Are these just occasional voltage spikes that are very fast in nature, or simple wide variances in the operating voltage.
There are plenty of 'Surge suppressor' devices available (check eBay), or metal-oxide varistor (MOV) if you intend
to work on the circuit itself. (for voltage spikes)
If these are long-term wide variances in operating voltages, you're going to need a more substantial voltage stabilizer system that you'll have to buy commercially.
So. What's the exact nature of your voltage problem?
