1. When you gig, is the guitar sitting on a stand beforehand, as well as during setbreaks, still pluged in? Just as Dennis said, a cable plugged into the guitar counts as "on" so you need to account for those moments in your overall use. FWIW, I always unplug ...always.
2. Are you using a good alkaline such as Duracell or Energizer?
If you're good with the above, then yessir, you've definitely got something wrong going on with the preamp.
Edward