Reving 300 E at idle?

my 87 300 E started reving at iddle yesterday. It reves back and forth as if someone was pushing on the accelerator and letting off every couple of seconds. What could possibly cause this. I recently gave it new plugs (which I do every 3000 miles because of oil fouling due to leaky valve guides). I put in my normal bosch suppers, and gave her an oil change. I've had nothing but problems since. Help please.

Danka.