Diagnosing & Fixing a Ground Problem

[ggking7]

I'm having a real problem with electrical noise in my power lines. The noise itself isn't audible but my stereo sounds much, much worse in the afternoon than in the morning or evening so I think this must be due to electrical noise coming in on the power line and entering my amp which is my only stereo component. Connecting the amp to a Tripp Lite isolation transformer keeps the sound consistent all day, but overall the sound is somewhat degraded compared to plugging it into the wall.

I've disconnected the line that powers my amp from all other outlets, connected all noisy appliances (refrigerator, computers) to isolation transformers to prevent them from dumping noise into the power line, shut off all unused circuit breakers, and plugged parallel filters into unused outlets in the apartment. Each of these things helps significantly but sound quality still plummets every afternoon.

I've been told the problem may be as severe as it is due to a grounding problem. How can I test for a grounding problem and if one is found, how is it fixed? I'm in a rented apartment and assuming the ground is just weak and non entirely non-existent, my landlord won't be interested in fixing it.

[Speedskater]

Normally grounding problems don't get worse each afternoon.
It might be a "lost neutral" problem in your or your neighbors apartment, but it would take some skill to diagnose.
How experienced are you with testing power line voltages?