Not sure exactly, I see the dark gray, gray, white then #xxxxxxxx.00C01570 error and sometimes only black screen.
I measured pin3 and seems it's fine.
That's potentially a different problem then, the one I described affects things much earlier on, meaning you'll get the guru message before the colour change.
Just to clarify, does this only happen following a brief on-off-on power cycle? It could also be a problem that the reset on power on isn't holding the system in reset state for long enough.
Keep in mind that the 5V supply has to fall low enough to trigger a reset in the first place, which would be to at least 1-2V at a guess. If the power interruption is too brief, the supply brownout can be enough to cause system operation to fail, but not to trigger the reset circuit, meaning it'll need a soft reset, or a longer power cycle, to restore operation. Which is why later designs used a proper supply monitor, or made the power supply do this - which is the purpose of the PG (power good) output from power supplies in A3000, A4000, etc.
You'd need to find a better way of testing it in your A500. Easiest way is to get a dual trace oscilloscope, set the time base to 100mSec/division, connect Ch A to the 5V supply and trigger on the rising edge, connect Ch B to pin 3 of the 555 timer IC.
What you'd expect to see is that on power up (when the 5V supply becomes active), timer pin 3 goes high almost immediately, stays high for approx. 470mSec, then goes low (bringing the system out of reset state).
If it's doing that, but the time is much shorter than expected, then there's probably a problem with capacitor C712 (10µF).
