Ive had the same problem. After a few days of use, my 99s started to feel mushy and not precise. I thought it was just me. But now ,i know it was the strings losening, because i own two 99s, and switched off to the one with the fresh string job,(same strings, tension) and it felt crisp and easy to play again.
But i also learned to work with the mushy one, as it seemed to be a matter of feel. Still prefer the fresh one, and i will get the mushy one restrung soon.
poly @ 61#, level 5.0