catalytic converter? please help!

I have a 1999 New Jetta GLS. I have been having an ongoing sporatic problem with hesitation and rough running. It seems as though the problem occurs when the gas is low (1/4 tank or less), and the traffic is stop and go. The car will start to run VERY rough, the engine light will come on and flash for a few miles, but if I get the car on the freeway and open it up, the problem goes away. The engine light will then go solid, and a few days later it will turn off.

Thoughts?!! Help!!!

It doesn't sound like a catalytic converter problem. Try getting the codes scanned as it will save alot of time. I think most Autozone stores now keep basic scannners & will read/reset for free.