I think my Amiga (and modem?) is hyper-sensitive to E/M noise! What to do?

[Zac67]

The caps in the PSU and/or onboard may have gone bad. They age and lose capacitance, thus lose their power to protect the logic from mains ripple.

I'd try another PSU to see if that changes anything. If it doesn't (or not sufficiently), check (bulging/leakage)and replace all bad and all large capacitors on the mainboard. A pain but these things do age.

[Zac67]

mrad wrote:

Conducted emissions can come through the power lines.  It seems unlikely to me that conducted emissions would affect both your modem & your Amiga though.

Hate to disagree, but a flaky PSU can very well cause fluctuations in the serial control or data lines, ranging from mis-handshaking to data loss, causing transmission interruptions or PPP errors...

Get a multimeter, set it to AC(!) in 0.2 to 2 V range and check the 5V and 12V power lines, at best on the mainboard, while switching the lights. Obviously the power should be DC only and AC component should be zero. If you do get (intermittent) readings above some few mV, the power is flaky.