I have an '87 560SEL that has occasionally starting running very rich. It will be running perfectly, idle speed 650rpm
Then just as quickly drop to 500rpm and loads up on gas. It almost appears as though I'm temporarily losing my electronic control over the fuel injection system. Any ideas as to what's going on here? Am I losing an engine control module or something?

If this is happening while idling, it could mean a check on the idle valve control. Start with this.