I only ever have problems with wildly fluctuating scores (high or low) when I am not playing regularly and don't have a good feel of which of my throws are high and low percentage.
To me, everything that happens on the course comes down to statistics. You have some shots you make 20% of the time, some you make 99% of the time, and a lot that are somewhere in between. If your normal round includes taking, say ten 70% shots then on a normal round you will make seven of them and miss three. Since a round of 18 holes is nowhere near large enough a sample size to expect the average value to be true or close to true all of the time you will most likely have some rounds where you make only three of those shots and some where you make all 10. The difference between those two rounds can be the difference between +5 and -2 at the end, and that is only on those 10 shots. Add in your odds across all drives, upshots, and putts across the round and if you are shooting low-percentage shots often you will see yourself wildly fluctuate between "on" games where you are unstoppable and "off" games where you are terrible. It is easy to have "on" games way more often when you are only shooting 90% shots.
Definitely take a dump before you play, and definitely don't be overthinking things since that will negatively affect performance. Those things are both absolutely true. But I also know that a lot of disc golfers struggling to be "in the zone" are throwing a lot of throws they aren't as good at as they think they are, and consequently have "on" games where they hit everything, "off" games where they miss everything, and a lot of mediocre games in between. If you play at least weekly on the same course and your scores are frequently more than 2-3 strokes apart (depending on the course, I could easily see a punishing course like the USDGC setup being much much bigger than that) your problem is more likely your shot selection rather than mental state.
This is all of course coming from the assumption that you like minimizing your score. I derive a lot of the pleasure I get from disc golf from the constant grind of self improvement. If you get more enjoyment from pulling off big, important shots then you might just have to settle for less consistency, although you can certainly strive to make all of your shots higher percentage. If a certain throw works for you sometimes but not always, you can practice it thousands of times until you are making it a very high percentage of the time. If you do this with all of your throws you will be consistent and really really good.
