First off, this will void your warranty. However, my keyboard is out of warranty and I experienced the A sharp key going a little velocity-indiscriminate again. Here is what I did to clean the contacts and restore it back to original performance:
Step 1: Read this guy's thread. His board isn't a PX-5S but pretty much everything he says still applies, the internals are similar.
Step 2. Assemble the required tools:
#2 Phillips screwdriver (you will definitely want one