Is it common for a check engine light to go on and off a lot?

I have a 2004 Chevrolet Impala with the 3.4 V6 with around 76k on it? Back in November 20th, 2010, my check engine light came on. I went to two auto parts stores and they both gave me an EGR code, saying that my EGR valve is faulty. Since then that light goes off and on (probably 6 or 7 times that light came on, went off, came back on, went off). At first it went off for 3 weeks, then it came back on and after that is gas been coming on and off. When the light comes on I am driving the car (it doesn't come on when I first start it). Some times it only comes on for 2 days then it goes off again. As of now the light is off. I put 2 bottles of STP fuel injector cleanrer in my tank (somebody that knows about cars told me to do that), and it seems to help. Okay, is it really my EGR valve acting up or something else like a sensor? Is it bad to drive around with a faulty EGR valve?