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

[DrDekker]

The light and fan switching on/off can certainly cause spikes, especially if the light and fan use a fair bit of current and the switch contacts are dirty/burnt.

As for the interference from motorbikes, that's usually down to the metal supressor casing being removed from the sparkplug caps (to reduce electrical shorts in wet weather), although I'd have thought that this type of interference would only affect aerial/satellite based reception/transmission.  Perhaps someone more knowledgeable can shed more light on that issue.

It could just be that you've got some dodgy cable shielding.  

[DrDekker]

Are you using the original Amiga PSU?  If so, try a different PSU, or better still, adapt an AT (200W-ish) PSU.

Interference from motorbikes aside, it maybe that a.c. is getting through to your mobo - perhaps part of the rectification has broken down in the PSU?

I had this once on one of my A600's.  It caused all sorts of weird effects - HD errors, graphical glitches, sound distortion/oscillations, to name but a few.  A 'new' AT supply sorted it out.