The evap purging is a different symptom and you will see the fuel trims go severly negative for no apperent reason which is really just the pcm compensating for the excessive fuel tank vapors that are being expelled into the engine. When the above mentioned condition happens the car will have perfect fuel trims driving down the road, in town, highway, it doesnt matter, and then out of the blue you will notice that the fuel trims start climbing up until they max at +16.4 and is never a negative number. It will stay locked at +16.4 indefinately until you give it a lot of throttle quickly or as Ryan mentioned above a high speed coast down. For the majority of the people that I have talked to about this problem their cars were stalling coming up to a stop light or it will badly surge and then cough out.
