That could be multiple things. However if still it has the original black AVG chip, replace it IMMEDIATELY, as it is a ticking time bomb, and will very likely take out your monitor when it goes (not if it goes, but when). Replacements below. This should be done regardless.
http://www.arcadeshop.com/i/956/replacement-atari-avg.htm
You could also have a separate digital (or possibly even analog) issue. However if it's intermittent like you have seen, it's more likely digital. Start with the AVG. If that doesn't fix it and you want to send it in, feel free to PM me anytime (see my sig below).