Check the voltages on the external floppy connector.

Also, try to gently slap the left side of the mac when you are sitting in front of it, tiny broken solder joints in the connector going to the logic board might cause that issue I believe. A re-solder/re-flow of any connections in the area of the power connector and switch are a good idea, but beware of the filter caps as they can contain high voltages.

Now my plus once did the same thing, and after I fiddled around a bit with the things above, it never did it again.