I'm also experiencing exactly the same problem.

It seems to occur when a certain amount of polyphony is applied.
You can make the keyboard hang just by pressing several keys using a sound like the Strings or Voices.

The only thing wrong I could spot by openning the keyboard was the battery on the CPU board that was leaking. Replacing it didn't solve the problem however.

Did you make any progress on your side ?

Cheers,
Daniel