I'm the exact same in the guitar department, play a riff for 5 mins flawlessly, try to record it as a 10 second clip (or even try to put it on a looper), fuck up a couple notes in.
I also interview badly frequently but not due to poor test performance but due to my demeanor in an inherently awkward situation.
As I perform more and more, I realize the hard part is getting out of my own way. I fumble and stutter playing something when I try to think about what I'm doing, trying too hard to make it good.
I'm at my best when my conscious mind doesn't really know what I'm doing, and I trust the musical muscle to do what I feel which produces more emotional, more compelling music
I also interview badly frequently but not due to poor test performance but due to my demeanor in an inherently awkward situation.