Most likely cause is the tuners, but pay close attention to the nut as well. If a string has begun to cut down into the nut material, it will cause the string to have more resistance to sliding. So if you bend the g, when you release the string it might not snap back to its normal position. Since the guitar you have is 3+3 tuning configuration, the d and g strings are the longest between the nut and the tuners allowing for more "spring" in those strings. Some of these guitars didn't use the best material for the nut since they weren't intended to be around for 20+ years. The likelyhood of body heat causing warpage (IMHO) is low since the only part of your body that contacts the neck is your hand, and its constantly moving. Ambient temp swings of +/- 10 degrees can certainly cause issues with tuning, but I don't think this is the case with your guitar. To try and figure out if the nut is the problem, follow these steps. Take off the strings and inspect the string grooves to make sure there's no debris or residue in the slots. Years of playing get skin oils, sweat, and dirt in the slots. Use a nylon bristled brush and alcohol to clean out the slots if necessary. Once clean, use a very light oil (like 3:1 or reel oil, NOT WD-40) and dab it on the nut. You don't need a lot, and less is definitely better than more. Put the strings back on and see if it makes a difference. If you find that the nut was the problem, you might think about investing in a higher quality one from StewMac or Allparts. Sometimes the nut material is fine, but they did a crappy job of cutting the slots. Any competent guitar tech should be able to recut the slots for improved performance. I like the bone nuts, but everyone has their own preference. Of course you will need to find someone who can install it for you properly. Hope this helps.