bigtiny

do you do a standard warmup before practicing/playing? It sounds like your suffering chop fatigue. This happens to me about 10 minutes after I start playing if I don't warmup. I've heard stories of guys who don't (need to) warm up. I'm not one of them....perhaps you aren't either.

Try this:

1) play major scale from low F# to C in the staff - long tones, slow, quiet
2) rest
3) starting on low E (bottom of the staff) do half and quarter note lip slurs up a perfect fourth 3 times. Do this starting on E and up to C# and back (E-A,F-Bb,F#-B, etc.)
4) rest
5) starting on low F#, play chromatic scale in eighth notes (medium tempo) up and and down and octave, then move up by half steps (F#, G, G#, etc.) until starting on C in the middle of the staff.
6) rest

Now try playing and see if you still have the fatigue problem....

bigtiny