I don't have any definitive suggestions, but I've got plenty of questions - maybe some will help...

Do you have any devices around that might produce IR signals, or anything connected to the 990's RS-232 port? That might help explain the unrequested volume changes.

Are you using the 990's included remote, or are you using a third party remote? (If it's a third party remote, the volume problem could be related to the way the volume commands were learned.)

The sluggish responsiveness suggests problems with the unit receiving IR. If there's an IR repeater attached and the unit can still see signals directly from the remote, the two nearly simultaneous signals could confuse matters a bit.

As for the complete lack of audio, I've got a couple possibly odd-ball thoughts. First, there should be a red indicator on the front panel immediately to the right of the input label - that's the indicator that says the main zone is active. If the mute button is held down for several seconds, the main zone is shut down and the headphone output is engaged. The even more odd-ball thought is to make sure the amp (or amps) is on.