I think it is your monitor. I had a Philips CM8833 which did just the same. A half hour into using it (after it had heated up), it would turn purple. Sometimes a really hard whack on the side would fix it for a while then it would turn purple again. Most likely the green gun on the monitor has an intermittent problem and is on the way out.
The only way to be sure it to try your A3000 on a different monitor or on a TV. If it still happens on a different monitor, try stripping your machine down and rebuilding it, cleaning the contacts on your zorro slots and any expansion cards, and re-seating any socketed chips.
