I keep my '68 MVI in good repair with a great set of techs. But this issue has happened several times. I pull it out for a big gig, and it plays awesome, I remember why I keep it. But then, after a couple hours practice, it starts to get difficult in the low register. I took it to a gig recently and regretted it. From experience, I'll store it, and pull it back out and it will play whisper easy again, but then get difficult. Whats going on? Keys are tight, pads are good (I volunteer to rebuild it, but the techs say its fine). I want to play it more. Moisture related? Thermal related? Any ideas?