Bought another Junker

[zeno]

The symptom is a power supply problem. Try running it on 12 VDC
& see if it clears up. If you are already using 12 VDC the wall wart may
be too light to run it.
The switch may also be related. Try some contact cleaner in it & turn it
on & off a bunch of times. Some of these sets run on 120V AC, 12 VDC,
batteries & rechargeable batteries so the on/off SW may be more than
a simple SPST.

73 Zeno
LFOD !