cuttig out when driving

i have a 2000 expedition w/ 4.6. i've changed spark plugs, and it seems since i've done that it been running like it's missing. the wierd thing is that it doesn't do it on a regular basis. the check engine light comes on then it blinks when i get on it. my boss has a tahoe and he said his was doing something similar, and the shop told him that his cateletic converter was pluged up a little. i am hearing a pinnging noise coming from the converter. could this be my problem? any help is greatly appreciated. thank you.

To test the cat put a vacuum gauge on and take a reading. 17 or below indicates a plugged cat. Make sure the spark plug boots are on fully.