[Ancient Geek]

Also, check all grounding points. Is there any shielding in the unit? Is so, check that it is grounded securely. If there is no shielding, check for proximity to RFI sources. This sounds like a very intermittent problem, such that odd noise spikes from, say, ill-behaved led lights or wall warts (not necessarily the one driving the unit) or even wifi routers, in combination with flaky design and/or flaky psu components could easily be at fault.