I have come across almost the exact same problem om my classic a few days back...

except that it very very rarely comes up by itself without the machine actually reading from the hdd or floppy.

it stays on for like 3 seconds and then everything is good again.

i have NOT yet recapped the analog board, i did recap the logic board with success.

i also want to know what is causing this... could it also be a bourns filter in my case?