Many things could be at work here. Here are a few questions to ask yourself: Is my thumb comfortably centered on the back of the neck, and not wrapped around it? Is my fretting hand relaxed? Is my bass well-balanced, so that my left hand isn't supporting the weight of the headstock while I play (test this by letting go with your fretting hand. If the bass drops down, then you're asking your hand to do double duty holding it up). Are you a very small person using a very long scale? Your bass could be a bad fit. If action were the issue, you would have more trouble on the higher frets than the lower, so that's not a clear answer either. The gauge of strings you use may be an issue, but I would look at these other issues first - 45 - 105 is medium gauge, so no harm in going lighter. I'm guesssing bass balance when standing is the likeliest place to look. Having "neck dive" can be very uncomfortable over hours of playing. Let us know what you think.