Volume increasing and decreasing on preamp

Your lost remote is in your chair. Once in a while you sit just right...or wrong...

Try a piece of tape over the IR receiver

Sounds like stray IR signals. Could be from any number of
sources.

You could try a "Factory Reset". From the manual:

Sometimes a factory reset is necessary in order to clear
problematic symptoms sometimes experienced in the
field.

A factory reset will clear all system settings, and all
presets. A factory reset will also clear any information
backed up onto the EEPROM that may have been
accomplished by a Memory Backup.

Simultaneously press and release the ON/STANDBY,
DOWN and LEVEL buttons. Once you see "Format
Filesys" on the front panel you can release the buttons.

A Factory Reset will erase the memory backup.
FACTORY RESET will take approximately 60 seconds to
execute

There is also a way to do an "IR LOCK". Since you're not
using the remote, I'd lock the IR sensor. The manual says
the procedure is under the Advanced Security Menu.