I think I have an amp problem

In the last day or so I’ve noticed an oddity that I think is an amp issue. I run a Plinius 8200 integrated amp bought in 2001 and every so often the sound loses its edge and clarity and seems to “step back” toward the front wall (behind the speakers). Sometimes it comes back toward the listener within a couple of minutes or even 5-10 seconds. Sometimes it almost seems like an imaginary speaker in the center is slowly turning - so I get the full force of the music only when the “speaker” faces me and the diminished sound as it turns away. When it happen ns I lose the fullness of sound, the life of the music. It happens on LPs and hi-def streaming but haven’t noticed it in CD - and I’ve been unable to make it happen on demand. Sorry if this explanation is clumsy — any insight appreciated. Know what this is? Cause? Anyone else experience it? Any way to fix it? Thanks