I have had a similar one with the Chopin Bb minor Prelude (Op. 28, No. 16). Forty four measures of solid 16ths at Presto con Fuoco!
I get the same thing when i play my tech. If I actually take a few seconds to relax myself, drop my shoulders, and let my hands rest on the keys, I always do way better.
The other thing that I noticed just now when I was playing that Handel is when I tense up, I not only bring my shoulders up (learning to relax those when horseback riding makes all the difference as well), but also I can feel my calves and my butt muscles tightening. Making the conscious effort to relax legs and butt helps with shoulders and arms.
Indeed, this is true. Out of curiosity, do you observe any other areas that are tense? In particular, watch out for your breathing, clenched jaws and a tensed neck (the neck is commonly the first area that tenses up, before anyhting else). Now, try to be aware of your entire body while you are playing, as well as listening to the sound you produce. It is difficult, but feasible when attentively practiced slowly with time (so I've been told )