HP330 intermittent keyboard sound

[twant4]

We bought a used HP330 quite a few years ago and it has been great except for having to replace keys. However, it has began to act up. First it will POP and then the keyboard output stops. If I hook up an external input, it will play through the speakers just fine. If we turn it off for a while, it may play again with no decisive time of when the pop is going to happen again, sometimes in a few minutes, sometimes in 30 or 40. My family really likes the piano so if someone has an easy check or seen this before an can suggest a part to shotgun in, it would be much appreciated.