Hey Ryan the shifting doesn't have much to do with the battery voltage, your tranny internally is starting to go bye bye ... especially considering the fluid was changed 3 months ago, the frciction plates are worn in my opionion. The old fluid had some of the friction particles floating around in it but now the new fluid is to slippery. I went through 3 auto trannys in my old 1g before I converted it to a manual. I had a very similar problem and 3 months later tranny wouldn't shift at all.