Do you have this problem as well, perhaps on another guitar? It's good one day, but terrible the next? What's going on here?
If your guitar feels/ plays differently from one day to the next - and you aren't changing the tuning - chances are, the neck bow (a.k.a neck relief) is fluctuating.
What is neck bow/ relief and why is it fluctuating?
Neck relief is the amount of bow (or lack of) in the neck itself- as best show in the diagram below.
The diagrams show the differences you may encounter with the amount of relief in your neck. With most guitars, you can adjust the amount of relief by tightening or loosening the truss rod. The truss rod is a steel rod or set of rods that are set in and run the length of the neck. There is often an adjustment nut at one end or the other. Ideally, your guitar will have just a slight bow in the neck - to accommodate string movement with your particular setup- and to minimize buzzing while playing. How to adjust your truss rod is covered in an older post at http://www.learn-guitarsetups.com/repair-blog/category/truss%20rod.
So why is it fluctuating?
My first bet is that the relative humidity in the air is fluctuating, and the neck- being sensitive to the changes of humidity- is quick to respond. From small gap (good), to no gap (buzzy and bad), to big gap (high string height and out of tune)- there are guitars out there that will fluctuate through these extremes on a weekly or even daily basis. Being that every piece of wood is different, some guitars are more sensitive than others. So if you have one that is misbehaving, fluctuating humidity is likely the reason why (unless you have a mischievous child or two in the house....).
Learn all about guitar setups in our book "How To Setup Your Guitar Like a Pro: An Easy Guide For Beginners", available through Amazon.com or with the Kindle version through Amazon.ca.