I don't see how it could be a reed problem. Far more likely to be condensation in the mpc, as reedscraper said. I vaguely remember having this happen with a specific metal mpc I had years ago, but I haven't had it happen with some other metal mpcs, and never with the HR mpcs I use now.
p.s. Now that I re-read the OP, she mentions that it was more of a problem with a specific reed. If that's the case, and it doesn't happen with other reeds, I'd just toss the one that causes the problem. It still seems odd to me that a reed could be the issue.